diff --git a/src/FiscalOS.API/Accounts/Connect/Endpoint.cs b/src/FiscalOS.API/Accounts/Connect/Endpoint.cs
index 1085541..099fbdb 100644
--- a/src/FiscalOS.API/Accounts/Connect/Endpoint.cs
+++ b/src/FiscalOS.API/Accounts/Connect/Endpoint.cs
@@ -10,9 +10,41 @@ internal static class Endpoint
   }
 
   private static async Task<IResult> HandleAsync(
-    [FromBody] Request request
+    HttpContext httpContext,
+    [FromBody] Request request,
+    [FromServices] AppDbContext appDbContext
   )
   {
+    var userId = httpContext.GetUserId();
+
+    var user = await appDbContext.Users
+      .Include(u => u.Institutions
+        .Where(
+          i => i.Metadata is PlaidMetadata &&
+            ((PlaidMetadata)i.Metadata).PlaidId == request.PlaidInstitutionId
+        )
+      )
+      .ThenInclude(i => i.Metadata)
+      .SingleOrDefaultAsync(u => u.Id == userId);
+
+    if (user is null)
+    {
+      return Results.Problem(
+        statusCode: StatusCodes.Status401Unauthorized,
+        title: "Unauthorized",
+        detail: "You are not authorized to connect an institution. Please log in and try again."
+      );
+    }
+
+    if (user.Institutions.Any())
+    {
+      return Results.Problem(
+        statusCode: StatusCodes.Status409Conflict,
+        title: "Institution already connected",
+        detail: "The user has already connected an institution with the provided Plaid Institution ID."
+      );
+    }
+
     return Results.Ok();
   }
 }
\ No newline at end of file
diff --git a/src/FiscalOS.API/Usings.cs b/src/FiscalOS.API/Usings.cs
index cf0e489..3ff8349 100644
--- a/src/FiscalOS.API/Usings.cs
+++ b/src/FiscalOS.API/Usings.cs
@@ -8,6 +8,7 @@ global using FiscalOS.API.Http;
 global using FiscalOS.API.Login;
 global using FiscalOS.API.Refresh;
 global using FiscalOS.Core.Authentication;
+global using FiscalOS.Infra.Accounts.Plaid;
 global using FiscalOS.Infra.Authentication;
 global using FiscalOS.Infra.Data;
 global using FiscalOS.Infra.DependencyInjection;
diff --git a/tests/FiscalOS.API.Tests/Integration/ConnectTests.cs b/tests/FiscalOS.API.Tests/Integration/ConnectTests.cs
index 57b2888..d3df9d7 100644
--- a/tests/FiscalOS.API.Tests/Integration/ConnectTests.cs
+++ b/tests/FiscalOS.API.Tests/Integration/ConnectTests.cs
@@ -1,3 +1,6 @@
+using FiscalOS.Core.Accounts;
+using FiscalOS.Infra.Accounts.Plaid;
+
 namespace FiscalOS.API.Tests.Integration;
 
 public class ConnectTests(TestApi testApi) : IntegrationTest(testApi)
@@ -80,4 +83,71 @@ public class ConnectTests(TestApi testApi) : IntegrationTest(testApi)
       ["PlaidInstitutionId"] = ["The PlaidInstitutionId field is required."],
     });
   }
+
+  [Fact]
+  public async Task Connect_WhenCalledWithNonExistentUser_ItShouldReturn401WithProblemDetails()
+  {
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, Guid.NewGuid().ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      publicToken = "token",
+      plaidInstitutionId = "id",
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, ConnectUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeProblemDetails(HttpStatusCode.Unauthorized);
+  }
+
+  [Fact]
+  public async Task Connect_WhenCalledWithPlaidInstitutionIdThatIsAlreadyConnected_ItShouldReturn409WithProblemDetails()
+  {
+    var (user, institution) = await ExecuteDbContextAsync(static async (context, sp) =>
+    {
+      var passwordHasher = sp.GetRequiredService<IPasswordHasher>();
+      var encryptor = sp.GetRequiredService<IEncryptor>();
+
+      var userEncryptionKey = await encryptor.GenerateEncryptedKeyAsync(TestContext.Current.CancellationToken);
+      var user = User.From("User1", passwordHasher.Hash("@Password1"), userEncryptionKey);
+
+      var encryptedAccessToken = await encryptor.EncryptAsyncFor(user, "accessToken", TestContext.Current.CancellationToken);
+      var plaidMetadata = PlaidMetadata.From("alreadyExists", "Some Bank", encryptedAccessToken);
+      var institution = Institution.From("Some Bank", plaidMetadata);
+
+      user.AddInstitution(institution);
+
+      await context.AddAsync(user, TestContext.Current.CancellationToken);
+      await context.SaveChangesAsync(TestContext.Current.CancellationToken);
+      return (user, institution);
+    });
+
+    var jwt = JwtTokenBuilder.New()
+      .WithClaim(JwtRegisteredClaimNames.Sub, user.Id.ToString())
+      .Build();
+
+    var json = JsonSerializer.Serialize(new
+    {
+      publicToken = "token",
+      plaidInstitutionId = ((PlaidMetadata)institution.Metadata).PlaidId,
+    });
+    using var content = new StringContent(json, Encoding.UTF8, "application/json");
+    using var request = new HttpRequestMessage(HttpMethod.Post, ConnectUri)
+    {
+      Content = content,
+    };
+    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", jwt);
+
+    var response = await Client.SendAsync(request, TestContext.Current.CancellationToken);
+
+    await response.Should().BeProblemDetails(HttpStatusCode.Conflict);
+  }
 }
\ No newline at end of file