diff --git a/test/test_functional.py b/test/test_functional.py
index a8e28f523a..1943b813d1 100644
--- a/test/test_functional.py
+++ b/test/test_functional.py
@@ -172,7 +172,7 @@ def test_istft_of_zeros(self):
     def test_istft_requires_overlap_windows(self):
         # the window is size 1 but it hops 20 so there is a gap which throw an error
         stft = torch.zeros((3, 5, 2))
-        self.assertRaises(AssertionError, torchaudio.functional.istft, stft, n_fft=4,
+        self.assertRaises(RuntimeError, torchaudio.functional.istft, stft, n_fft=4,
                           hop_length=20, win_length=1, window=torch.ones(1))
 
     def test_istft_requires_nola(self):
@@ -192,11 +192,11 @@ def test_istft_requires_nola(self):
         # A window of ones meets NOLA but a window of zeros does not. This should
         # throw an error.
         torchaudio.functional.istft(stft, **kwargs_ok)
-        self.assertRaises(AssertionError, torchaudio.functional.istft, stft, **kwargs_not_ok)
+        self.assertRaises(RuntimeError, torchaudio.functional.istft, stft, **kwargs_not_ok)
 
     def test_istft_requires_non_empty(self):
-        self.assertRaises(AssertionError, torchaudio.functional.istft, torch.zeros((3, 0, 2)), 2)
-        self.assertRaises(AssertionError, torchaudio.functional.istft, torch.zeros((0, 3, 2)), 2)
+        self.assertRaises(RuntimeError, torchaudio.functional.istft, torch.zeros((3, 0, 2)), 2)
+        self.assertRaises(RuntimeError, torchaudio.functional.istft, torch.zeros((0, 3, 2)), 2)
 
     def _test_istft_of_sine(self, amplitude, L, n):
         # stft of amplitude*sin(2*pi/L*n*x) with the hop length and window size equaling L
diff --git a/torchaudio/functional.py b/torchaudio/functional.py
index 2245768b99..bf5665adfb 100644
--- a/torchaudio/functional.py
+++ b/torchaudio/functional.py
@@ -2,6 +2,7 @@
 
 import math
 from typing import Optional, Tuple
+import warnings
 
 import torch
 from torch import Tensor
@@ -49,7 +50,7 @@ def istft(
         win_length: Optional[int] = None,
         window: Optional[Tensor] = None,
         center: bool = True,
-        pad_mode: str = "reflect",
+        pad_mode: Optional[str] = None,
         normalized: bool = False,
         onesided: bool = True,
         length: Optional[int] = None,
@@ -94,8 +95,7 @@ def istft(
         center (bool, optional): Whether ``input`` was padded on both sides so
             that the :math:`t`-th frame is centered at time :math:`t \times \text{hop\_length}`.
             (Default: ``True``)
-        pad_mode (str, optional): Controls the padding method used when ``center`` is True. (Default:
-            ``"reflect"``)
+        pad_mode: This argument was ignored and to be removed.
         normalized (bool, optional): Whether the STFT was normalized. (Default: ``False``)
         onesided (bool, optional): Whether the STFT is onesided. (Default: ``True``)
         length (int or None, optional): The amount to trim the signal by (i.e. the
@@ -104,105 +104,16 @@ def istft(
     Returns:
         Tensor: Least squares estimation of the original signal of size (..., signal_length)
     """
-    stft_matrix_dim = stft_matrix.dim()
-    assert 3 <= stft_matrix_dim, "Incorrect stft dimension: %d" % (stft_matrix_dim)
-    assert stft_matrix.numel() > 0
-
-    if stft_matrix_dim == 3:
-        # add a channel dimension
-        stft_matrix = stft_matrix.unsqueeze(0)
-
-    # pack batch
-    shape = stft_matrix.size()
-    stft_matrix = stft_matrix.reshape(-1, shape[-3], shape[-2], shape[-1])
-
-    dtype = stft_matrix.dtype
-    device = stft_matrix.device
-    fft_size = stft_matrix.size(1)
-    assert (onesided and n_fft // 2 + 1 == fft_size) or (
-        not onesided and n_fft == fft_size
-    ), (
-        "one_sided implies that n_fft // 2 + 1 == fft_size and not one_sided implies n_fft == fft_size. "
-        + "Given values were onesided: %s, n_fft: %d, fft_size: %d"
-        % ("True" if onesided else False, n_fft, fft_size)
-    )
-
-    # use stft defaults for Optionals
-    if win_length is None:
-        win_length = n_fft
-
-    if hop_length is None:
-        hop_length = int(win_length // 4)
-
-    # There must be overlap
-    assert 0 < hop_length <= win_length
-    assert 0 < win_length <= n_fft
-
-    if window is None:
-        window = torch.ones(win_length, device=device, dtype=dtype)
-
-    assert window.dim() == 1 and window.size(0) == win_length
-
-    if win_length != n_fft:
-        # center window with pad left and right zeros
-        left = (n_fft - win_length) // 2
-        window = torch.nn.functional.pad(window, (left, n_fft - win_length - left))
-        assert window.size(0) == n_fft
-    # win_length and n_fft are synonymous from here on
-
-    stft_matrix = stft_matrix.transpose(1, 2)  # size (channel, n_frame, fft_size, 2)
-    stft_matrix = torch.irfft(
-        stft_matrix, 1, normalized, onesided, signal_sizes=(n_fft,)
-    )  # size (channel, n_frame, n_fft)
-
-    assert stft_matrix.size(2) == n_fft
-    n_frame = stft_matrix.size(1)
-
-    ytmp = stft_matrix * window.view(1, 1, n_fft)  # size (channel, n_frame, n_fft)
-    # each column of a channel is a frame which needs to be overlap added at the right place
-    ytmp = ytmp.transpose(1, 2)  # size (channel, n_fft, n_frame)
-
-    # this does overlap add where the frames of ytmp are added such that the i'th frame of
-    # ytmp is added starting at i*hop_length in the output
-    y = torch.nn.functional.fold(
-        ytmp, (1, (n_frame - 1) * hop_length + n_fft), (1, n_fft), stride=(1, hop_length)
-    ).squeeze(2)
-
-    # do the same for the window function
-    window_sq = (
-        window.pow(2).view(n_fft, 1).repeat((1, n_frame)).unsqueeze(0)
-    )  # size (1, n_fft, n_frame)
-    window_envelop = torch.nn.functional.fold(
-        window_sq, (1, (n_frame - 1) * hop_length + n_fft), (1, n_fft), stride=(1, hop_length)
-    ).squeeze(2)  # size (1, 1, expected_signal_len)
-
-    expected_signal_len = n_fft + hop_length * (n_frame - 1)
-    assert y.size(2) == expected_signal_len
-    assert window_envelop.size(2) == expected_signal_len
-
-    half_n_fft = n_fft // 2
-    # we need to trim the front padding away if center
-    start = half_n_fft if center else 0
-    end = -half_n_fft if length is None else start + length
-
-    y = y[:, :, start:end]
-    window_envelop = window_envelop[:, :, start:end]
-
-    # check NOLA non-zero overlap condition
-    window_envelop_lowest = window_envelop.abs().min()
-    assert window_envelop_lowest > 1e-11, "window overlap add min: %f" % (
-        window_envelop_lowest
-    )
-
-    y = (y / window_envelop).squeeze(1)  # size (channel, expected_signal_len)
-
-    # unpack batch
-    y = y.reshape(shape[:-3] + y.shape[-1:])
-
-    if stft_matrix_dim == 3:  # remove the channel dimension
-        y = y.squeeze(0)
-
-    return y
+    warnings.warn(
+        'istft has been moved to PyTorch and will be removed from torchaudio, '
+        'please use torch.istft instead.')
+    if pad_mode is not None:
+        warnings.warn(
+            'The parameter `pad_mode` was ignored in isftft, and is thus being deprecated. '
+            'Please set `pad_mode` to None to suppress this warning.')
+    return torch.istft(
+        input=stft_matrix, n_fft=n_fft, hop_length=hop_length, win_length=win_length, window=window,
+        center=center, normalized=normalized, onesided=onesided, length=length)
 
 
 def spectrogram(