diff --git a/oidc/config_test.go b/oidc/config_test.go
index 4dd3b00..85b576f 100644
--- a/oidc/config_test.go
+++ b/oidc/config_test.go
@@ -7,6 +7,7 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
+	"net/http"
 	"testing"
 	"time"
 
@@ -44,6 +45,8 @@ func TestNewConfig(t *testing.T) {
 		return time.Now().Add(-1 * time.Minute)
 	}
 
+	testRt := newTestRoundTripper(t)
+
 	type args struct {
 		issuer              string
 		clientID            string
@@ -61,7 +64,7 @@ func TestNewConfig(t *testing.T) {
 		wantErrContains string
 	}{
 		{
-			name: "valid-with-all-valid-opts",
+			name: "valid-with-all-valid-opts-except-with-round-tripper",
 			args: args{
 				issuer:              "http://your_issuer/",
 				clientID:            "your_client_id",
@@ -103,6 +106,49 @@ func TestNewConfig(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "with-round-tripper",
+			args: args{
+				issuer:              "http://your_issuer/",
+				clientID:            "your_client_id",
+				clientSecret:        "your_client_secret",
+				supported:           []Alg{RS512},
+				allowedRedirectURLs: []string{"http://your_redirect_url", "http://redirect_url_two", "http://redirect_url_three"},
+				opt: []Option{
+					WithAudiences("your_aud1", "your_aud2"),
+					WithScopes("email", "profile"),
+					WithRoundTripper(testRt),
+					WithNow(testNow),
+					WithProviderConfig(&ProviderConfig{
+						AuthURL:     "https://auth-endpoint",
+						JWKSURL:     "https://jwks-endpoint",
+						TokenURL:    "https://token-endpoint",
+						UserInfoURL: "https://userinfo-endpoint",
+					}),
+				},
+			},
+			want: &Config{
+				Issuer:               "http://your_issuer/",
+				ClientID:             "your_client_id",
+				ClientSecret:         "your_client_secret",
+				SupportedSigningAlgs: []Alg{RS512},
+				Audiences:            []string{"your_aud1", "your_aud2"},
+				Scopes:               []string{oidc.ScopeOpenID, "email", "profile"},
+				RoundTripper:         testRt,
+				NowFunc:              testNow,
+				AllowedRedirectURLs: []string{
+					"http://your_redirect_url",
+					"http://redirect_url_two",
+					"http://redirect_url_three",
+				},
+				ProviderConfig: &ProviderConfig{
+					AuthURL:     "https://auth-endpoint",
+					JWKSURL:     "https://jwks-endpoint",
+					TokenURL:    "https://token-endpoint",
+					UserInfoURL: "https://userinfo-endpoint",
+				},
+			},
+		},
 		{
 			name: "missing-provider-config-auth-url",
 			args: args{
@@ -282,6 +328,22 @@ func TestNewConfig(t *testing.T) {
 			wantErr:   true,
 			wantIsErr: ErrInvalidCACert,
 		},
+		{
+			name: "invalid-both-cert-and-round-tripper",
+			args: args{
+				issuer:              "http://your_issuer/",
+				clientID:            "your_client_id",
+				clientSecret:        "your_client_secret",
+				supported:           []Alg{RS512},
+				allowedRedirectURLs: []string{"http://your_redirect_url"},
+				opt: []Option{
+					WithProviderCA(testCaPem),
+					WithRoundTripper(testRt),
+				},
+			},
+			wantErr:   true,
+			wantIsErr: ErrInvalidParameter,
+		},
 		{
 			name: "invalid-alg",
 			args: args{
@@ -430,6 +492,7 @@ func TestConfig_Hash(t *testing.T) {
 		require.NoError(t, err)
 		return c
 	}
+	testRt := newTestRoundTripper(t)
 	tests := []struct {
 		name      string
 		c1        *Config
@@ -473,6 +536,42 @@ func TestConfig_Hash(t *testing.T) {
 			),
 			wantEqual: true,
 		},
+		{
+			name: "equal-with-round-tripper",
+			c1: newCfg(
+				"https://www.alice.com",
+				"client-id", "client-secret",
+				[]Alg{RS256},
+				[]string{"www.alice.com/callback", "www.bob.com/callback"},
+				WithScopes("email", "profile"),
+				WithAudiences("alice.com", "bob.com"),
+				WithRoundTripper(testRt),
+				WithNow(time.Now),
+				WithProviderConfig(&ProviderConfig{
+					AuthURL:     "https://auth-endpoint",
+					JWKSURL:     "https://jwks-endpoint",
+					TokenURL:    "https://token-endpoint",
+					UserInfoURL: "https://userinfo-endpoint",
+				}),
+			),
+			c2: newCfg(
+				"https://www.alice.com",
+				"client-id", "client-secret",
+				[]Alg{RS256},
+				[]string{"www.bob.com/callback", "www.alice.com/callback"},
+				WithScopes("profile", "email"),
+				WithAudiences("bob.com", "alice.com"),
+				WithRoundTripper(testRt),
+				WithNow(time.Now),
+				WithProviderConfig(&ProviderConfig{
+					AuthURL:     "https://auth-endpoint",
+					JWKSURL:     "https://jwks-endpoint",
+					TokenURL:    "https://token-endpoint",
+					UserInfoURL: "https://userinfo-endpoint",
+				}),
+			),
+			wantEqual: true,
+		},
 		{
 			name: "diff-issuer",
 			c1: newCfg(
@@ -664,6 +763,29 @@ func TestConfig_Hash(t *testing.T) {
 			),
 			wantEqual: false,
 		},
+		{
+			name: "diff-round-trippers",
+			c1: newCfg(
+				"https://www.alice.com",
+				"client-id", "client-secret",
+				[]Alg{RS256},
+				[]string{"www.alice.com/callback"},
+				WithScopes("email", "profile"),
+				WithAudiences("alice.com", "bob.com"),
+				WithRoundTripper(newTestRoundTripper(t)),
+				WithNow(time.Now),
+			),
+			c2: newCfg(
+				"https://www.alice.com",
+				"client-id", "client-secret",
+				[]Alg{RS256},
+				[]string{"www.alice.com/callback"},
+				WithScopes("email", "profile"),
+				WithAudiences("alice.com", "bob.com"),
+				WithNow(time.Now),
+			),
+			wantEqual: false,
+		},
 		{
 			name: "diff-now-func",
 			c1: newCfg(
@@ -855,3 +977,20 @@ func TestConfig_Hash(t *testing.T) {
 		})
 	}
 }
+
+type testRoundTripper struct {
+	transport http.RoundTripper
+	called    int
+}
+
+func newTestRoundTripper(t *testing.T) *testRoundTripper {
+	t.Helper()
+	return &testRoundTripper{
+		transport: http.DefaultTransport,
+	}
+}
+
+func (rt *testRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	rt.called++
+	return rt.transport.RoundTrip(req)
+}
diff --git a/oidc/provider_test.go b/oidc/provider_test.go
index b715863..8dca1dd 100644
--- a/oidc/provider_test.go
+++ b/oidc/provider_test.go
@@ -714,6 +714,31 @@ func TestHTTPClient(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, c.Transport, p.client.Transport)
 	})
+	t.Run("check-transport-with-round-tripper", func(t *testing.T) {
+		testRt := newTestRoundTripper(t)
+		p := &Provider{
+			config: &Config{
+				RoundTripper: testRt,
+			},
+		}
+		c, err := p.HTTPClient()
+		require.NoError(t, err)
+		assert.Equal(t, c.Transport, p.client.Transport)
+	})
+	t.Run("err-both-ca-and-round-trippe", func(t *testing.T) {
+		_, testCaPem := TestGenerateCA(t, []string{"localhost"})
+
+		p := &Provider{
+			config: &Config{
+				ProviderCA:   testCaPem,
+				RoundTripper: newTestRoundTripper(t),
+			},
+		}
+		_, err := p.HTTPClient()
+		require.Error(t, err)
+		assert.ErrorIs(t, err, ErrInvalidParameter)
+		assert.ErrorContains(t, err, "you cannot specify config for both a ProviderCA and Transport")
+	})
 }
 
 func TestProvider_UserInfo(t *testing.T) {