diff --git a/src/pages/accounting/AccountingReconcileDetailPage.tsx b/src/pages/accounting/AccountingReconcileDetailPage.tsx
index 82da21d..b43b384 100644
--- a/src/pages/accounting/AccountingReconcileDetailPage.tsx
+++ b/src/pages/accounting/AccountingReconcileDetailPage.tsx
@@ -18,7 +18,7 @@ import {
   Select, SelectContent, SelectItem, SelectTrigger, SelectValue, SelectGroup, SelectLabel,
 } from "@/components/ui/select";
 import { Table, TableHeader, TableRow, TableHead, TableBody, TableCell } from "@/components/ui/table";
-import { ArrowLeft, CheckCircle2, AlertTriangle, FileDown, Search, Loader2, ArrowUp, ArrowDown, ChevronsUpDown, Plus, Ban, Pencil } from "lucide-react";
+import { ArrowLeft, CheckCircle2, AlertTriangle, FileDown, Search, Loader2, ArrowUp, ArrowDown, ChevronsUpDown, Plus, Ban, Pencil, RotateCcw } from "lucide-react";
 import { cn } from "@/lib/utils";
 import { toast } from "sonner";
 import { money, fmtDate } from "./lib/format";
@@ -512,6 +512,38 @@ export default function AccountingReconcileDetailPage() {
     qc.invalidateQueries({ queryKey: ["transactions", cid] });
   };
 
+  // Undo a completed reconciliation: reopen its items (clear flag + link removed)
+  // and delete the reconciliation record so the period can be reconciled again.
+  // Only the most recent reconciliation may be undone — undoing an earlier one
+  // would invalidate the opening balance every later period was built on.
+  const [undoingId, setUndoingId] = useState<string | null>(null);
+  const undoReconciliation = async (h: any) => {
+    if (!window.confirm(
+      `Undo the reconciliation for ${fmtDate(h.statement_end_date)}?\n\n` +
+      `Its ${money(h.statement_balance, cur)} statement will be removed and all items it cleared will be reopened so you can reconcile the period again. This does not delete any transactions.`
+    )) return;
+    setUndoingId(h.id);
+    try {
+      const { error: txErr } = await accounting
+        .from("transactions")
+        .update({ cleared: false, reconciliation_id: null })
+        .eq("reconciliation_id", h.id);
+      if (txErr) { toast.error(txErr.message); return; }
+      const { error: recErr } = await accounting.from("reconciliations").delete().eq("id", h.id);
+      if (recErr) { toast.error(recErr.message); return; }
+      toast.success("Reconciliation undone — the period is open again.");
+      qc.invalidateQueries({ queryKey: ["recon-history", accountId] });
+      qc.invalidateQueries({ queryKey: ["recon-txs", accountId] });
+      qc.invalidateQueries({ queryKey: ["recon-last", cid] });
+      qc.invalidateQueries({ queryKey: ["recon-accounts", cid] });
+      qc.invalidateQueries({ queryKey: ["account", accountId] });
+    } finally {
+      setUndoingId(null);
+    }
+  };
+  // The single most-recent completed reconciliation (history is sorted desc).
+  const latestCompletedId = (history as any[]).find((h: any) => h.status === "completed")?.id ?? null;
+
   if (!associationId) return <p className="text-sm text-muted-foreground">Select an association.</p>;
   if (companyLoading) return <div className="flex justify-center py-12"><Loader2 className="h-6 w-6 animate-spin text-muted-foreground" /></div>;
   if (companyError || !companyId) return <p className="text-sm text-muted-foreground text-center py-12">{companyError || "Accounting setup is not ready."}</p>;
@@ -781,6 +813,21 @@ export default function AccountingReconcileDetailPage() {
                           }}>
                             <FileDown className="h-4 w-4 mr-1" />View Report
                           </Button>
+                          {h.status === "completed" && h.id === latestCompletedId && (
+                            <Button
+                              size="sm"
+                              variant="ghost"
+                              className="text-destructive hover:text-destructive"
+                              disabled={undoingId === h.id}
+                              onClick={() => undoReconciliation(h)}
+                              title="Reopen this period and remove the reconciliation"
+                            >
+                              {undoingId === h.id
+                                ? <Loader2 className="h-4 w-4 mr-1 animate-spin" />
+                                : <RotateCcw className="h-4 w-4 mr-1" />}
+                              Undo
+                            </Button>
+                          )}
                         </TableCell>
                       </TableRow>
                     ))}