diff --git a/client/src/components/TaskDetailsModal.tsx b/client/src/components/TaskDetailsModal.tsx
index 4f7cfca..36679e1 100644
--- a/client/src/components/TaskDetailsModal.tsx
+++ b/client/src/components/TaskDetailsModal.tsx
@@ -13,7 +13,7 @@ import { Popover, PopoverContent, PopoverTrigger } from '@/components/ui/popover
 import { cn } from "@/lib/utils";
 import { apiRequest, queryClient } from "@/lib/queryClient";
 import { useQuery } from '@tanstack/react-query';
-import { Clock, Timer, FileText, Plus, Edit2, Save, X, Calendar as CalendarIcon, Tag, Link2, Check, LayoutList, Trash2, ArrowRight } from 'lucide-react';
+import { Clock, Timer, FileText, Plus, Edit2, Save, X, Calendar as CalendarIcon, Tag, Link2, Check, LayoutList, Trash2, ArrowRight, CheckCircle2 } from 'lucide-react';
 import { Calendar } from '@/components/ui/calendar';
 import { format } from 'date-fns';
 import { de, enUS } from 'date-fns/locale';
@@ -75,7 +75,7 @@ export default function TaskDetailsModal({
   const [notes, setNotes] = useState('');
   const [isEditingNotes, setIsEditingNotes] = useState(false);
 
-  // Time reporting state  
+  // Time reporting state
   const [isAddingTime, setIsAddingTime] = useState(false);
   const [hours, setHours] = useState(0);
   const [minutes, setMinutes] = useState(0);
@@ -83,6 +83,13 @@ export default function TaskDetailsModal({
   const [timeDescription, setTimeDescription] = useState('');
   const [activeTimeTab, setActiveTimeTab] = useState('clock');
 
+  // Mark as done flow state
+  const [showTimeRequiredDialog, setShowTimeRequiredDialog] = useState(false);
+  const [markDoneHours, setMarkDoneHours] = useState(0);
+  const [markDoneMinutes, setMarkDoneMinutes] = useState(0);
+  const [markDoneManualHours, setMarkDoneManualHours] = useState('');
+  const [markDoneTimeTab, setMarkDoneTimeTab] = useState('clock');
+
   // Time entries state - simulate from timeTracked for display
   const [timeEntries, setTimeEntries] = useState<TimeEntry[]>([]);
 
@@ -130,9 +137,86 @@ export default function TaskDetailsModal({
     setMinutes(0);
     setManualHours('');
     setTimeDescription('');
+    setShowTimeRequiredDialog(false);
+    setMarkDoneHours(0);
+    setMarkDoneMinutes(0);
+    setMarkDoneManualHours('');
     onClose();
   };
 
+  // Handle Mark as Done button click
+  const handleMarkAsDone = () => {
+    if (!task) return;
+
+    // Check if time has been tracked
+    if (!task.timeTracked || task.timeTracked === 0) {
+      // Show dialog to enter time first
+      setShowTimeRequiredDialog(true);
+      setMarkDoneHours(0);
+      setMarkDoneMinutes(0);
+      setMarkDoneManualHours('');
+      setMarkDoneTimeTab('clock');
+    } else {
+      // Time already tracked, mark as done directly
+      completeTaskAsDone();
+    }
+  };
+
+  // Complete the task as done
+  const completeTaskAsDone = () => {
+    if (!task) return;
+
+    const updatedTask: Task = {
+      ...task,
+      title: editedTitle,
+      description: editedDescription || null,
+      status: 'done',
+      priority: editedPriority,
+      labelId: editedLabelId,
+      dueDate: editedDueDate ? new Date(editedDueDate) : null,
+      startDate: editedStartDate ? new Date(editedStartDate) : null,
+      estimatedDuration: editedEstimatedDuration ?? null,
+      dependencies: editedDependencies
+    };
+    onSave(updatedTask);
+    handleClose();
+  };
+
+  // Handle saving time and then marking as done
+  const handleSaveTimeAndMarkDone = () => {
+    if (!task) return;
+
+    let totalMinutes = 0;
+
+    if (markDoneTimeTab === 'clock') {
+      totalMinutes = markDoneHours * 60 + markDoneMinutes;
+    } else {
+      const parsedHours = parseFloat(markDoneManualHours) || 0;
+      totalMinutes = Math.round(parsedHours * 60);
+    }
+
+    if (totalMinutes <= 0) return;
+
+    // Create updated task with time and done status
+    const updatedTask: Task = {
+      ...task,
+      title: editedTitle,
+      description: editedDescription || null,
+      status: 'done',
+      priority: editedPriority,
+      labelId: editedLabelId,
+      dueDate: editedDueDate ? new Date(editedDueDate) : null,
+      startDate: editedStartDate ? new Date(editedStartDate) : null,
+      estimatedDuration: editedEstimatedDuration ?? null,
+      dependencies: editedDependencies,
+      timeTracked: (task.timeTracked || 0) + totalMinutes
+    };
+
+    onSave(updatedTask);
+    setShowTimeRequiredDialog(false);
+    handleClose();
+  };
+
   const handleSaveTaskEdit = () => {
     if (!task) return;
 
@@ -322,6 +406,7 @@ export default function TaskDetailsModal({
   const isSubtask = !!task.parentTaskId;
 
   return (
+    <>
     <Dialog open={isOpen} onOpenChange={handleClose}>
       <DialogContent className="w-[95vw] max-w-lg sm:max-w-2xl max-h-[90vh] overflow-y-auto p-4 sm:p-6">
         <DialogHeader>
@@ -717,7 +802,7 @@ export default function TaskDetailsModal({
                     </Popover>
                   </div>
 
-                  {/* Save Button */}
+                  {/* Save and Mark as Done Buttons */}
                   <div className="flex gap-2 pt-2">
                     <Button
                       onClick={handleSaveTaskEdit}
@@ -727,6 +812,18 @@ export default function TaskDetailsModal({
                       <Save className="w-4 h-4 mr-1" />
                       {t('taskDetails.save')}
                     </Button>
+                    {editedStatus !== 'done' && (
+                      <Button
+                        onClick={handleMarkAsDone}
+                        disabled={!editedTitle.trim()}
+                        variant="default"
+                        className="bg-green-600 hover:bg-green-700"
+                        data-testid="button-mark-as-done"
+                      >
+                        <CheckCircle2 className="w-4 h-4 mr-1" />
+                        {t('taskDetails.markAsDone', 'Mark as Done')}
+                      </Button>
+                    )}
                   </div>
                 </div>
               </Card>
@@ -1013,6 +1110,139 @@ export default function TaskDetailsModal({
           </div>
         </div>
       </DialogContent>
-    </Dialog >
+    </Dialog>
+
+      {/* Time Required Dialog - shown when marking as done without time */}
+      <Dialog open={showTimeRequiredDialog} onOpenChange={setShowTimeRequiredDialog}>
+        <DialogContent className="w-[95vw] max-w-md">
+          <DialogHeader>
+            <DialogTitle className="flex items-center gap-2">
+              <Clock className="w-5 h-5" />
+              {t('taskDetails.timeRequired', 'Time Required')}
+            </DialogTitle>
+          </DialogHeader>
+
+          <div className="space-y-4">
+            <p className="text-sm text-muted-foreground">
+              {t('taskDetails.timeRequiredDesc', 'Please enter the time spent on this task before marking it as done.')}
+            </p>
+
+            <Tabs value={markDoneTimeTab} onValueChange={setMarkDoneTimeTab}>
+              <TabsList className="grid w-full grid-cols-2">
+                <TabsTrigger value="clock">
+                  <Clock className="w-4 h-4 mr-2" />
+                  {t('taskDetails.clock', 'Clock')}
+                </TabsTrigger>
+                <TabsTrigger value="manual">
+                  <Timer className="w-4 h-4 mr-2" />
+                  {t('taskDetails.manual', 'Manual')}
+                </TabsTrigger>
+              </TabsList>
+
+              <TabsContent value="clock" className="space-y-4">
+                <div className="space-y-4">
+                  <div className="text-center">
+                    <div className="text-2xl font-mono">
+                      {formatTime(markDoneHours, markDoneMinutes)}
+                    </div>
+                  </div>
+
+                  <div className="grid grid-cols-2 gap-4">
+                    {/* Hours Picker */}
+                    <div className="space-y-2">
+                      <label className="text-sm font-medium text-center block">{t('taskDetails.hours', 'Hours')}</label>
+                      <div className="space-y-1 max-h-32 overflow-y-auto border rounded-md p-2">
+                        {Array.from({ length: 13 }, (_, i) => (
+                          <Button
+                            key={i}
+                            variant={markDoneHours === i ? "default" : "ghost"}
+                            size="sm"
+                            className="w-full"
+                            onClick={() => setMarkDoneHours(i)}
+                          >
+                            {i}
+                          </Button>
+                        ))}
+                      </div>
+                    </div>
+
+                    {/* Minutes Picker */}
+                    <div className="space-y-2">
+                      <label className="text-sm font-medium text-center block">{t('taskDetails.minutes', 'Minutes')}</label>
+                      <div className="space-y-1 max-h-32 overflow-y-auto border rounded-md p-2">
+                        {Array.from({ length: 12 }, (_, i) => i * 5).map((minute) => (
+                          <Button
+                            key={minute}
+                            variant={markDoneMinutes === minute ? "default" : "ghost"}
+                            size="sm"
+                            className="w-full"
+                            onClick={() => setMarkDoneMinutes(minute)}
+                          >
+                            {minute}
+                          </Button>
+                        ))}
+                      </div>
+                    </div>
+                  </div>
+                </div>
+              </TabsContent>
+
+              <TabsContent value="manual" className="space-y-4">
+                <div className="space-y-3">
+                  <div className="text-center">
+                    <div className="text-2xl font-mono">
+                      {(() => {
+                        const parsedHours = parseFloat(markDoneManualHours) || 0;
+                        const h = Math.floor(parsedHours);
+                        const m = Math.round((parsedHours - h) * 60);
+                        return formatTime(h, m);
+                      })()}
+                    </div>
+                  </div>
+
+                  <div>
+                    <label className="text-sm font-medium block mb-2">{t('taskDetails.hoursDecimal', 'Hours (decimal)')}</label>
+                    <Input
+                      type="number"
+                      step="0.25"
+                      min="0"
+                      max="24"
+                      placeholder="e.g., 1.5 for 1 hour 30 minutes"
+                      value={markDoneManualHours}
+                      onChange={(e) => setMarkDoneManualHours(e.target.value)}
+                    />
+                    <p className="text-xs text-muted-foreground mt-1">
+                      {t('taskDetails.timeExamples', 'Examples: 0.5 = 30min, 1.25 = 1h 15min')}
+                    </p>
+                  </div>
+                </div>
+              </TabsContent>
+            </Tabs>
+
+            <div className="flex gap-2 pt-2">
+              <Button
+                onClick={handleSaveTimeAndMarkDone}
+                disabled={
+                  markDoneTimeTab === 'clock'
+                    ? markDoneHours === 0 && markDoneMinutes === 0
+                    : !markDoneManualHours.trim() || parseFloat(markDoneManualHours) <= 0
+                }
+                className="flex-1 bg-green-600 hover:bg-green-700"
+                data-testid="button-save-time-and-done"
+              >
+                <CheckCircle2 className="w-4 h-4 mr-1" />
+                {t('taskDetails.saveAndMarkDone', 'Save & Mark as Done')}
+              </Button>
+              <Button
+                variant="outline"
+                onClick={() => setShowTimeRequiredDialog(false)}
+              >
+                {t('common.cancel', 'Cancel')}
+              </Button>
+            </div>
+          </div>
+        </DialogContent>
+      </Dialog>
+    </>
   );
 }
\ No newline at end of file
diff --git a/client/src/i18n/locales/de.json b/client/src/i18n/locales/de.json
index 857c468..a2a219e 100644
--- a/client/src/i18n/locales/de.json
+++ b/client/src/i18n/locales/de.json
@@ -197,7 +197,15 @@
     "durationUnits": {
       "m": "Min",
       "h": "Std"
-    }
+    },
+    "markAsDone": "Als erledigt markieren",
+    "timeRequired": "Zeitangabe erforderlich",
+    "timeRequiredDesc": "Bitte geben Sie die für diese Aufgabe aufgewendete Zeit ein, bevor Sie sie als erledigt markieren.",
+    "hours": "Stunden",
+    "minutes": "Minuten",
+    "hoursDecimal": "Stunden (dezimal)",
+    "timeExamples": "Beispiele: 0.5 = 30 Min, 1.25 = 1 Std 15 Min",
+    "saveAndMarkDone": "Speichern & Als erledigt markieren"
   },
   "taskCard": {
     "play": "Timer starten",
diff --git a/client/src/i18n/locales/en.json b/client/src/i18n/locales/en.json
index 6678dcd..9874a64 100644
--- a/client/src/i18n/locales/en.json
+++ b/client/src/i18n/locales/en.json
@@ -195,7 +195,15 @@
     "durationUnits": {
       "m": "m",
       "h": "h"
-    }
+    },
+    "markAsDone": "Mark as Done",
+    "timeRequired": "Time Required",
+    "timeRequiredDesc": "Please enter the time spent on this task before marking it as done.",
+    "hours": "Hours",
+    "minutes": "Minutes",
+    "hoursDecimal": "Hours (decimal)",
+    "timeExamples": "Examples: 0.5 = 30min, 1.25 = 1h 15min",
+    "saveAndMarkDone": "Save & Mark as Done"
   },
   "taskCard": {
     "play": "Start timer",