diff --git a/qiskit_experiments/curve_analysis/guess.py b/qiskit_experiments/curve_analysis/guess.py
index ddf2617bba..1d377d2038 100644
--- a/qiskit_experiments/curve_analysis/guess.py
+++ b/qiskit_experiments/curve_analysis/guess.py
@@ -398,7 +398,9 @@ def rb_decay(
     y = y[valid_inds]
     x = x[valid_inds]
 
-    if len(x) < 2:
+    if len(x) == 0:
+        return 0
+    if len(x) == 1:
         # If number of element is 1, assume y(0) = 1.0 and directly compute alpha.
         a = 1.0 - b
         return ((y[0] - b) / a) ** (1 / x[0])
diff --git a/releasenotes/notes/fix-guess-rb-decay-f78e40a7d6d8dd67.yaml b/releasenotes/notes/fix-guess-rb-decay-f78e40a7d6d8dd67.yaml
new file mode 100644
index 0000000000..ea9a49cad7
--- /dev/null
+++ b/releasenotes/notes/fix-guess-rb-decay-f78e40a7d6d8dd67.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a bug in :func:`~.rb_decay` where it unintentionally raises
+    an ``IndexError`` if all ``y`` values are below ``b`` value
+    so that it returns ``0`` for the case.
diff --git a/test/curve_analysis/test_guess.py b/test/curve_analysis/test_guess.py
index bff8870628..e111e2a7fc 100644
--- a/test/curve_analysis/test_guess.py
+++ b/test/curve_analysis/test_guess.py
@@ -207,3 +207,10 @@ def test_rb_decay(self, a, b, alpha):
         alpha_guess = guess.rb_decay(x, y, b=b)
 
         self.assertAlmostEqual(alpha, alpha_guess, delta=alpha * 0.1)
+
+    def test_rb_decay_with_very_bad_output(self):
+        """Test if rb decay guess does not raise an error even for very bad outputs."""
+        x = np.array([1, 2, 3])
+        y = np.array([0.24, 0.22, 0.23])  # all are below b
+        out = guess.rb_decay(x=x, y=y, b=0.25)
+        self.assertEqual(out, 0.0)