diff --git a/pkg/controller/api/username_check.go b/pkg/controller/api/username_check.go
new file mode 100644
index 0000000..66886d4
--- /dev/null
+++ b/pkg/controller/api/username_check.go
@@ -0,0 +1,57 @@
+package api
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"code.nonshy.com/nonshy/website/pkg/models"
+)
+
+// UsernameCheck API.
+func UsernameCheck() http.HandlerFunc {
+	// Request JSON schema.
+	type Request struct {
+		Username string `json:"username"`
+	}
+
+	// Response JSON schema.
+	type Response struct {
+		OK    bool   `json:"OK"`
+		Error string `json:"error,omitempty"`
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			SendJSON(w, http.StatusNotAcceptable, Response{
+				Error: "POST method only",
+			})
+			return
+		}
+
+		// Parse request payload.
+		var req Request
+		if err := ParseJSON(r, &req); err != nil {
+			SendJSON(w, http.StatusBadRequest, Response{
+				Error: fmt.Sprintf("Error with request payload: %s", err),
+			})
+			return
+		}
+
+		// Username to test.
+		var username = strings.TrimSpace(strings.ToLower(req.Username))
+
+		// Does it exist?
+		if _, err := models.FindUser(username); err == nil {
+			SendJSON(w, http.StatusOK, Response{
+				Error: "That username is already taken, please try another one.",
+			})
+			return
+		}
+
+		// Send success response.
+		SendJSON(w, http.StatusOK, Response{
+			OK: true,
+		})
+	})
+}
diff --git a/pkg/router/router.go b/pkg/router/router.go
index 11be908..6ee2ab4 100644
--- a/pkg/router/router.go
+++ b/pkg/router/router.go
@@ -93,6 +93,7 @@ func New() http.Handler {
 	// JSON API endpoints.
 	mux.HandleFunc("/v1/version", api.Version())
 	mux.HandleFunc("/v1/users/me", api.LoginOK())
+	mux.HandleFunc("/v1/users/check-username", api.UsernameCheck())
 	mux.Handle("/v1/likes", middleware.LoginRequired(api.Likes()))
 	mux.Handle("/v1/notifications/read", middleware.LoginRequired(api.ReadNotification()))
 	mux.Handle("/v1/notifications/delete", middleware.LoginRequired(api.ClearNotification()))
diff --git a/web/templates/account/signup.html b/web/templates/account/signup.html
index 79091a9..87ab0a4 100644
--- a/web/templates/account/signup.html
+++ b/web/templates/account/signup.html
@@ -108,6 +108,20 @@
                     value="{{.Username}}"
                     required>
                 <small class="has-text-grey">Usernames are 3 to 32 characters a-z 0-9 . -</small>
+
+                <!-- Username checking -->
+                <div class="notification is-info is-light py-2 px-4 mt-1"
+                    id="username-checking" style="display: none">
+                    <i class="fa fa-spinner fa-spin mr-1"></i> Checking username...
+                </div>
+                <div class="notification is-success is-light py-2 px-4 mt-1"
+                    id="username-ok" style="display: none">
+                    <i class="fa fa-check mr-1"></i> Looks good!
+                </div>
+                <div class="notification is-danger is-light py-2 px-4 mt-1"
+                    id="username-error" style="display: none">
+                    <i class="fa fa-xmark mr-1"></i> That username is already taken!
+                </div>
             </div>
             <div class="field">
                 <label class="label" for="password">Enter a passphrase:</label>
@@ -153,3 +167,61 @@
     </div>
 </div>
 {{end}}
+{{define "scripts"}}
+<script>
+window.addEventListener("DOMContentLoaded", (event) => {
+    // Set up username checking script.
+    const $username = document.querySelector("#username"),
+        $unCheck = document.querySelector("#username-checking"),
+        $unOK = document.querySelector("#username-ok"),
+        $unError = document.querySelector("#username-error");
+
+    let onChange = (e) => {
+        $unCheck.style.display = "block";
+        $unOK.style.display = "none";
+        $unError.style.display = "none";
+
+        if ($username.value.length < 3) {
+            $unCheck.style.display = "none";
+            return;
+        };
+
+        return fetch("/v1/users/check-username", {
+            method: "POST",
+            mode: "same-origin",
+            cache: "no-cache",
+            credentials: "same-origin",
+            headers: {
+                "Content-Type": "application/json",
+            },
+            body: JSON.stringify({
+                "username": $username.value,
+            }),
+        })
+        .then((response) => response.json())
+        .then((data) => {
+            if (data.StatusCode !== 200) {
+                window.alert(data.data.error);
+                return;
+            }
+
+            let result = data.data;
+            if (result.OK) {
+                $unOK.style.display = "block";
+                $unError.style.display = "none";
+            } else {
+                $unOK.style.display = "none";
+                $unError.style.display = "block";
+            }
+        }).catch(resp => {
+            window.alert(resp);
+        }).finally(() => {
+            $unCheck.style.display = "none";
+        })
+    };
+
+    $username.addEventListener("change", onChange);
+    $username.addEventListener("blur", onChange);
+});
+</script>
+{{end}}