diff --git a/cmd/verify/main.go b/cmd/verify/main.go
index 718cca5..67a6c87 100644
--- a/cmd/verify/main.go
+++ b/cmd/verify/main.go
@@ -2,7 +2,9 @@ package main
 
 import (
 	"context"
+	"encoding/csv"
 	"os"
+	"strings"
 
 	"github.com/rs/zerolog/log"
 
@@ -27,12 +29,22 @@ func main() {
 		firestoreProjectID = v
 	}
 
+	var extraCaCerts []string
+	if v, ok := os.LookupEnv("EXTRA_CA_CERTS"); ok {
+		var err error
+		extraCaCerts, err = csv.NewReader(strings.NewReader(v)).Read()
+		if err != nil {
+			log.Fatal().Err(err).Msg("failed to parse $EXTRA_CA_CERTS (expected comma-separated list of file paths)")
+		}
+	}
+
 	srv := verify.New(
 		verify.WithBindAddress(addr),
 		verify.WithFirestoreProjectID(firestoreProjectID),
 		verify.WithJWKSEndpoint(jwksEndpoint),
 		verify.WithExpectedJWTIssuer(os.Getenv("EXPECTED_JWT_ISSUER")),
 		verify.WithExpectedJWTAudience(os.Getenv("EXPECTED_JWT_AUDIENCE")),
+		verify.WithExtraCACerts(extraCaCerts...),
 	)
 	err := srv.Run(context.Background())
 	if err != nil {
diff --git a/config.go b/config.go
index 6036f73..3a71bfb 100644
--- a/config.go
+++ b/config.go
@@ -15,6 +15,7 @@ type config struct {
 	jwksEndpoint        string
 	expectedJWTIssuer   string
 	expectedJWTAudience string
+	extraCACerts        []string
 }
 
 // An Option customizes the config.
@@ -57,6 +58,15 @@ func WithFirestoreProjectID(projectID string) Option {
 	}
 }
 
+// WithExtraCACerts adds paths to custom CA certificates to the config.
+// Certificates added with this option will be used in addition to the system
+// default pool.
+func WithExtraCACerts(paths ...string) Option {
+	return func(cfg *config) {
+		cfg.extraCACerts = append(cfg.extraCACerts, paths...)
+	}
+}
+
 func getConfig(options ...Option) *config {
 	cfg := new(config)
 	WithBindAddress(DefaultBindAddress)(cfg)
diff --git a/tls.go b/tls.go
index 4945c06..531d06f 100644
--- a/tls.go
+++ b/tls.go
@@ -14,13 +14,21 @@ import (
 
 const maxRemoteWait = 5 * time.Second
 
+type tlsVerifierOptions struct {
+	rootCAs *x509.CertPool
+}
+
 type tlsVerifier struct {
+	tlsVerifierOptions
 	mu     sync.Mutex
 	errors map[string]error
 }
 
-func newTLSVerifier() *tlsVerifier {
-	return &tlsVerifier{errors: make(map[string]error)}
+func newTLSVerifier(opts tlsVerifierOptions) *tlsVerifier {
+	return &tlsVerifier{
+		tlsVerifierOptions: opts,
+		errors:             make(map[string]error),
+	}
 }
 
 func (v *tlsVerifier) DialTLSContext(ctx context.Context, network, addr string) (net.Conn, error) {
@@ -57,6 +65,7 @@ func (v *tlsVerifier) VerifyPeerCertificate(serverName string, rawCerts [][]byte
 
 	opts := x509.VerifyOptions{
 		DNSName:       serverName,
+		Roots:         v.rootCAs,
 		Intermediates: x509.NewCertPool(),
 	}
 	for _, cert := range certs[1:] {
diff --git a/verify.go b/verify.go
index 16b1e9a..78db123 100644
--- a/verify.go
+++ b/verify.go
@@ -2,8 +2,10 @@ package verify
 
 import (
 	"context"
+	"crypto/x509"
 	"net"
 	"net/http"
+	"os"
 
 	"cloud.google.com/go/firestore"
 	"github.com/go-chi/chi"
@@ -26,9 +28,30 @@ type Server struct {
 // New creates a new Server.
 func New(options ...Option) *Server {
 	cfg := getConfig(options...)
+
+	var verifierOpts tlsVerifierOptions
+	if len(cfg.extraCACerts) > 0 {
+		pool, err := x509.SystemCertPool()
+		if err != nil {
+			panic(err)
+		}
+		for _, certPath := range cfg.extraCACerts {
+			cert, err := os.ReadFile(certPath)
+			if err != nil {
+				log.Fatal().Err(err).Str("path", certPath).Msg("failed to read CA cert")
+			} else {
+				log.Info().Str("path", certPath).Msg("adding extra CA cert")
+			}
+			ok := pool.AppendCertsFromPEM(cert)
+			if !ok {
+				log.Warn().Str("path", certPath).Msg("no CA certs found in file")
+			}
+		}
+		verifierOpts.rootCAs = pool
+	}
 	return &Server{
 		cfg:         cfg,
-		tlsVerifier: newTLSVerifier(),
+		tlsVerifier: newTLSVerifier(verifierOpts),
 	}
 }