diff --git a/internal/api/server.go b/internal/api/server.go
index cf2aaf7..dfe1ac4 100644
--- a/internal/api/server.go
+++ b/internal/api/server.go
@@ -129,9 +129,12 @@ func (a *apiServer) routes() http.Handler {
 	r.Get("/{id}", a.handlePasteView)
 	r.Post("/{id}", a.handlePasteView)
 
-	r.NotFound(func(w http.ResponseWriter, r *http.Request) {
-		writeErr(w, 404, "not found")
-	})
+// #173: missing paste URLs render the main UI with a friendly not-found
+// message instead of a bare JSON 404. Known routes (above) handle real pages;
+// anything else is a nonexistent paste ID or typo.
+r.NotFound(func(w http.ResponseWriter, r *http.Request) {
+	a.webHandlers().HandleNotFoundPage(w, r)
+})
 	return r
 }
 
diff --git a/internal/web/templates/notfound.html b/internal/web/templates/notfound.html
new file mode 100644
index 0000000..eda487f
--- /dev/null
+++ b/internal/web/templates/notfound.html
@@ -0,0 +1,13 @@
+{{template "head" .}}
+{{template "topbar" .}}
+<div class="center">
+  <div class="float notfound-card">
+    <div class="inner">
+      <div class="lockring"><svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="12" cy="12" r="10"/><line x1="12" y1="8" x2="12" y2="12"/><line x1="12" y1="16" x2="12.01" y2="16"/></svg></div>
+      <h1>Paste ID not found</h1>
+      <p class="sub">The paste <span class="slug">/{{.ID}}</span> does not exist, has expired, or was burned.</p>
+      <a class="btn" href="/new">Create a new paste</a>
+    </div>
+  </div>
+</div>
+{{template "foot" .}}
diff --git a/internal/web/web.go b/internal/web/web.go
index 074d183..b502a4d 100644
--- a/internal/web/web.go
+++ b/internal/web/web.go
@@ -104,6 +104,13 @@ func (h *Handlers) renderPage(w http.ResponseWriter, name string, data any) {
 	}
 }
 
+// RenderNotFoundPage is the exported not-found renderer used by the api
+// package (#173): the router's NotFound handler renders the main UI with a
+// friendly "Paste ID not found" message, still with HTTP 404.
+func (h *Handlers) RenderNotFoundPage(w http.ResponseWriter, r *http.Request) {
+	h.renderNotFound(w, r, r.URL.Path)
+}
+
 // RenderPage is the exported wrapper used by the api package (#4 can pages).
 func (h *Handlers) RenderPage(w http.ResponseWriter, name string, data any) {
 	h.renderPage(w, name, data)
@@ -200,6 +207,30 @@ func (h *Handlers) writeRateLimited(w http.ResponseWriter, retryAfterSecs int) {
 	w.Write([]byte(`{"error":"rate limit exceeded"}`))
 }
 
+// renderNotFound serves the friendly not-found page (#173): the main UI
+// chrome (topbar, centered card) with a "Paste ID not found" message in the
+// result card, instead of a bare text 404. Still returns HTTP 404 so
+// crawlers/validators see the correct status.
+func (h *Handlers) renderNotFound(w http.ResponseWriter, r *http.Request, id string) {
+	h.renderPageStatus(w, "notfound.html", http.StatusNotFound, map[string]any{"Page": "notfound", "ID": id})
+}
+
+// HandleNotFoundPage serves the friendly not-found page for unknown routes
+// (#173): main UI chrome with a "Paste ID not found" message. Called from the
+// chi NotFound handler in the api package.
+func (h *Handlers) HandleNotFoundPage(w http.ResponseWriter, r *http.Request) {
+	h.renderNotFound(w, r, r.URL.Path)
+}
+
+// renderPageStatus renders a template with an explicit HTTP status code.
+func (h *Handlers) renderPageStatus(w http.ResponseWriter, name string, status int, data any) {
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.WriteHeader(status)
+	if err := h.UI.tmpl.ExecuteTemplate(w, name, data); err != nil {
+		http.Error(w, "template error: "+err.Error(), 500)
+	}
+}
+
 func (h *Handlers) renderPaste(w http.ResponseWriter, row *store.PasteRow, justCreated bool, deletionToken string, readsRemaining *int) {
 	lines := strings.Count(row.Content, "\n") + 1
 	gutter := ""
@@ -262,11 +293,14 @@ func (h *Handlers) HandlePasteView(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if row == nil {
-		http.NotFound(w, r)
+		// #173: a missing paste ID gets the main UI with a friendly message,
+		// not a bare text 404 page.
+		h.renderNotFound(w, r, id)
 		return
 	}
 	if row.ExpiresAt.Valid && row.ExpiresAt.Int64 < time.Now().Unix() {
-		http.Error(w, "paste expired", 404)
+		// #173: expired pastes render the same friendly not-found UI.
+		h.renderNotFound(w, r, id)
 		return
 	}
 	if row.PasswordHash.Valid {
@@ -333,8 +367,8 @@ func (h *Handlers) HandlePasteView(w http.ResponseWriter, r *http.Request) {
 	// Just-created first render does not count as a read for the creator.
 	if !justCreated {
 		rem, admitted := h.Store.RegisterRead(row, h.ViewerID(r), h.BurnWindowMin())
-		if !admitted { // #58: lost the burn claim; do not render content
-			http.NotFound(w, r)
+		if !admitted { // #58: lost the burn claim; #173: friendly not-found UI
+			h.renderNotFound(w, r, row.ID)
 			return
 		}
 		h.renderPaste(w, row, false, "", rem)