diff --git a/bundle/sign.go b/bundle/sign.go
index 13d6667f58..cf9a3e183a 100644
--- a/bundle/sign.go
+++ b/bundle/sign.go
@@ -6,6 +6,7 @@
 package bundle
 
 import (
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
 
@@ -76,7 +77,11 @@ func (*DefaultSigner) GenerateSignedToken(files []FileInfo, sc *SigningConfig, k
 		return "", err
 	}
 
-	token, err := jws.SignLiteral(payload, jwa.SignatureAlgorithm(sc.Algorithm), privateKey, hdr)
+	token, err := jws.SignLiteral(payload,
+		jwa.SignatureAlgorithm(sc.Algorithm),
+		privateKey,
+		hdr,
+		rand.Reader)
 	if err != nil {
 		return "", err
 	}
diff --git a/internal/jwx/jws/jws.go b/internal/jwx/jws/jws.go
index 6fca28d23c..bfa498bb0f 100644
--- a/internal/jwx/jws/jws.go
+++ b/internal/jwx/jws/jws.go
@@ -21,8 +21,10 @@ package jws
 
 import (
 	"bytes"
+	"crypto/rand"
 	"encoding/base64"
 	"encoding/json"
+	"io"
 	"strings"
 
 	"github.com/open-policy-agent/opa/internal/jwx/jwa"
@@ -37,7 +39,7 @@ import (
 // it in compact serialization format. In this format you may NOT use
 // multiple signers.
 //
-func SignLiteral(payload []byte, alg jwa.SignatureAlgorithm, key interface{}, hdrBuf []byte) ([]byte, error) {
+func SignLiteral(payload []byte, alg jwa.SignatureAlgorithm, key interface{}, hdrBuf []byte, rnd io.Reader) ([]byte, error) {
 	encodedHdr := base64.RawURLEncoding.EncodeToString(hdrBuf)
 	encodedPayload := base64.RawURLEncoding.EncodeToString(payload)
 	signingInput := strings.Join(
@@ -50,7 +52,14 @@ func SignLiteral(payload []byte, alg jwa.SignatureAlgorithm, key interface{}, hd
 	if err != nil {
 		return nil, errors.Wrap(err, `failed to create signer`)
 	}
-	signature, err := signer.Sign([]byte(signingInput), key)
+
+	var signature []byte
+	switch s := signer.(type) {
+	case *sign.ECDSASigner:
+		signature, err = s.SignWithRand([]byte(signingInput), key, rnd)
+	default:
+		signature, err = signer.Sign([]byte(signingInput), key)
+	}
 	if err != nil {
 		return nil, errors.Wrap(err, `failed to sign Payload`)
 	}
@@ -81,7 +90,9 @@ func SignWithOption(payload []byte, alg jwa.SignatureAlgorithm, key interface{})
 	if err != nil {
 		return nil, errors.Wrap(err, `failed to marshal Headers`)
 	}
-	return SignLiteral(payload, alg, key, hdrBuf)
+	// NOTE(sr): we don't use SignWithOption -- if we did, this rand.Reader
+	// should come from the BuiltinContext's Seed, too.
+	return SignLiteral(payload, alg, key, hdrBuf, rand.Reader)
 }
 
 // Verify checks if the given JWS message is verifiable using `alg` and `key`.
diff --git a/internal/jwx/jws/jws_test.go b/internal/jwx/jws/jws_test.go
index b5df98b31e..815210db86 100644
--- a/internal/jwx/jws/jws_test.go
+++ b/internal/jwx/jws/jws_test.go
@@ -261,7 +261,7 @@ func TestEncode(t *testing.T) {
 			t.Fatal("Failed to parse key")
 		}
 		var jwsCompact []byte
-		jwsCompact, err = jws.SignLiteral([]byte(examplePayload), alg, key, hdrBytes)
+		jwsCompact, err = jws.SignLiteral([]byte(examplePayload), alg, key, hdrBytes, rand.Reader)
 		if err != nil {
 			t.Fatal("Failed to sign message")
 		}
@@ -599,13 +599,13 @@ func TestSignErrors(t *testing.T) {
 		}
 	})
 	t.Run("Invalid signature algorithm", func(t *testing.T) {
-		_, err := jws.SignLiteral([]byte("payload"), jwa.SignatureAlgorithm("dummy"), nil, []byte("header"))
+		_, err := jws.SignLiteral([]byte("payload"), jwa.SignatureAlgorithm("dummy"), nil, []byte("header"), rand.Reader)
 		if err == nil {
 			t.Fatal("JWS signing should have failed")
 		}
 	})
 	t.Run("Invalid signature algorithm", func(t *testing.T) {
-		_, err := jws.SignLiteral([]byte("payload"), jwa.SignatureAlgorithm("dummy"), nil, []byte("header"))
+		_, err := jws.SignLiteral([]byte("payload"), jwa.SignatureAlgorithm("dummy"), nil, []byte("header"), rand.Reader)
 		if err == nil {
 			t.Fatal("JWS signing should have failed")
 		}
diff --git a/internal/jwx/jws/sign/ecdsa.go b/internal/jwx/jws/sign/ecdsa.go
index 7023906806..62af72b6c1 100644
--- a/internal/jwx/jws/sign/ecdsa.go
+++ b/internal/jwx/jws/sign/ecdsa.go
@@ -4,6 +4,7 @@ import (
 	"crypto"
 	"crypto/ecdsa"
 	"crypto/rand"
+	"io"
 
 	"github.com/open-policy-agent/opa/internal/jwx/jwa"
 
@@ -25,7 +26,7 @@ func init() {
 }
 
 func makeECDSASignFunc(hash crypto.Hash) ecdsaSignFunc {
-	return ecdsaSignFunc(func(payload []byte, key *ecdsa.PrivateKey) ([]byte, error) {
+	return ecdsaSignFunc(func(payload []byte, key *ecdsa.PrivateKey, rnd io.Reader) ([]byte, error) {
 		curveBits := key.Curve.Params().BitSize
 		keyBytes := curveBits / 8
 		// Curve bits do not need to be a multiple of 8.
@@ -34,7 +35,7 @@ func makeECDSASignFunc(hash crypto.Hash) ecdsaSignFunc {
 		}
 		h := hash.New()
 		h.Write(payload)
-		r, s, err := ecdsa.Sign(rand.Reader, key, h.Sum(nil))
+		r, s, err := ecdsa.Sign(rnd, key, h.Sum(nil))
 		if err != nil {
 			return nil, errors.Wrap(err, "failed to sign payload using ecdsa")
 		}
@@ -69,8 +70,9 @@ func (s ECDSASigner) Algorithm() jwa.SignatureAlgorithm {
 	return s.alg
 }
 
-// Sign signs payload with a ECDSA private key
-func (s ECDSASigner) Sign(payload []byte, key interface{}) ([]byte, error) {
+// SignWithRand signs payload with a ECDSA private key and a provided randomness
+// source (such as `rand.Reader`).
+func (s ECDSASigner) SignWithRand(payload []byte, key interface{}, r io.Reader) ([]byte, error) {
 	if key == nil {
 		return nil, errors.New(`missing private key while signing payload`)
 	}
@@ -79,6 +81,10 @@ func (s ECDSASigner) Sign(payload []byte, key interface{}) ([]byte, error) {
 	if !ok {
 		return nil, errors.Errorf(`invalid key type %T. *ecdsa.PrivateKey is required`, key)
 	}
+	return s.sign(payload, privateKey, r)
+}
 
-	return s.sign(payload, privateKey)
+// Sign signs payload with a ECDSA private key
+func (s ECDSASigner) Sign(payload []byte, key interface{}) ([]byte, error) {
+	return s.SignWithRand(payload, key, rand.Reader)
 }
diff --git a/internal/jwx/jws/sign/interface.go b/internal/jwx/jws/sign/interface.go
index 42a10c42e4..2ef2bee486 100644
--- a/internal/jwx/jws/sign/interface.go
+++ b/internal/jwx/jws/sign/interface.go
@@ -3,6 +3,7 @@ package sign
 import (
 	"crypto/ecdsa"
 	"crypto/rsa"
+	"io"
 
 	"github.com/open-policy-agent/opa/internal/jwx/jwa"
 )
@@ -28,7 +29,7 @@ type RSASigner struct {
 	sign rsaSignFunc
 }
 
-type ecdsaSignFunc func([]byte, *ecdsa.PrivateKey) ([]byte, error)
+type ecdsaSignFunc func([]byte, *ecdsa.PrivateKey, io.Reader) ([]byte, error)
 
 // ECDSASigner uses crypto/ecdsa to sign the payloads.
 type ECDSASigner struct {
diff --git a/plugins/rest/rest_auth.go b/plugins/rest/rest_auth.go
index 5151af439c..f6b5168371 100644
--- a/plugins/rest/rest_auth.go
+++ b/plugins/rest/rest_auth.go
@@ -219,7 +219,11 @@ func (ap *oauth2ClientCredentialsAuthPlugin) createAuthJWT(claims map[string]int
 		jwsHeaders = []byte(fmt.Sprintf(`{"typ":"JWT","alg":"%s"}`, ap.signingKey.Algorithm))
 	}
 
-	jwsCompact, err := jws.SignLiteral(payload, jwa.SignatureAlgorithm(ap.signingKey.Algorithm), signingKey, jwsHeaders)
+	jwsCompact, err := jws.SignLiteral(payload,
+		jwa.SignatureAlgorithm(ap.signingKey.Algorithm),
+		signingKey,
+		jwsHeaders,
+		rand.Reader)
 	if err != nil {
 		return nil, err
 	}
diff --git a/topdown/tokens.go b/topdown/tokens.go
index 67137b7220..64828831fd 100644
--- a/topdown/tokens.go
+++ b/topdown/tokens.go
@@ -851,7 +851,7 @@ func commonBuiltinJWTEncodeSign(bctx BuiltinContext, inputHeaders, jwsPayload, j
 
 	// process payload and sign
 	var jwsCompact []byte
-	jwsCompact, err = jws.SignLiteral([]byte(jwsPayload), alg, key, jwsHeaders)
+	jwsCompact, err = jws.SignLiteral([]byte(jwsPayload), alg, key, jwsHeaders, bctx.Seed)
 	if err != nil {
 		return err
 	}
diff --git a/topdown/tokens_test.go b/topdown/tokens_test.go
index 305b48b8a0..48c2d08e8f 100644
--- a/topdown/tokens_test.go
+++ b/topdown/tokens_test.go
@@ -445,3 +445,50 @@ func TestTopDownJWTEncodeSignES512(t *testing.T) {
 		t.Fatal("Failed to verify message")
 	}
 }
+
+// NOTE(sr): The stdlib ecdsa package will randomly read 1 byte from the source
+// and discard it: so passing a fixed-seed `rand.New(rand.Source(seed))` via
+// `rego.WithSeed` will not do the trick, the output would still randomly be
+// one of two possible signatures. To fix that for testing, we're reaching
+// deeper here, and use a "constant number generator". It doesn't matter if the
+// first byte is discarded, the second one looks just the same.
+type cng struct{}
+
+func (*cng) Read(p []byte) (int, error) {
+	for i := range p {
+		p[i] = 4
+	}
+	return len(p), nil
+}
+
+func TestTopdownJWTEncodeSignECWithSeedReturnsSameSignature(t *testing.T) {
+	query := `io.jwt.encode_sign({"alg": "ES256"},{"pay": "load"},
+	  {"kty":"EC",
+	   "crv":"P-256",
+	   "x":"f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU",
+	   "y":"x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0",
+	   "d":"jpsQnnGQmL-YBIffH1136cspYG6-0iY7X1fCE9-E9LI"
+	  }, x)`
+	encodedSigned := "eyJhbGciOiAiRVMyNTYifQ.eyJwYXkiOiAibG9hZCJ9.-LoHxtbT8t_TnqlLyONI4BtjvfkySO8TcoCFENqTTH2AKxvn29nAjxOdlbY-0EKVM2nJ4ukCx4IGtZtuwXr0VQ"
+
+	for i := 0; i < 10; i++ {
+		q := NewQuery(ast.MustParseBody(query)).
+			WithSeed(&cng{}).
+			WithStrictBuiltinErrors(true).
+			WithCompiler(ast.NewCompiler())
+
+		qrs, err := q.Run(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		} else if len(qrs) != 1 {
+			t.Fatal("expected exactly one result but got:", qrs)
+		}
+
+		if exp, act := 1, len(qrs); exp != act {
+			t.Fatalf("expected %d results, got %d", exp, act)
+		}
+		if exp, act := ast.String(encodedSigned), qrs[0][ast.Var("x")].Value; !exp.Equal(act) {
+			t.Fatalf("unexpected result: want %v, got %v", exp, act)
+		}
+	}
+}