diff --git a/firecrown/likelihood/_base.py b/firecrown/likelihood/_base.py
index 439709193..31b730cf9 100644
--- a/firecrown/likelihood/_base.py
+++ b/firecrown/likelihood/_base.py
@@ -32,6 +32,7 @@
 import sacc
 from pydantic import BaseModel, ConfigDict, Field
 from scipy.interpolate import Akima1DInterpolator
+from scipy.signal import fftconvolve
 
 from firecrown.data_types import DataVector, TheoryVector
 from firecrown.modeling_tools import ModelingTools
@@ -808,6 +809,7 @@ def apply(
 
 SOURCE_GALAXY_SYSTEMATIC_DEFAULT_DELTA_Z = 0.0
 SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_Z = 1.0
+SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_V = 0.0
 
 
 def dndz_shift_and_stretch_active(
@@ -1006,6 +1008,150 @@ def create_global(self) -> PhotoZShiftandStretch:
         raise ValueError("PhotoZShiftandStretch cannot be global.")
 
 
+def dndz_stretch_fog_gaussian(
+    z,
+    dndz,
+    sigma_v: float,
+) -> tuple[npt.NDArray[np.float64], npt.NDArray[np.float64]]:
+    """Apply the Fingers-of-God (FoG) effect on the spec-z distribution.
+
+    This function applies a gaussian kernel to model the FoG effect
+    due to velocity dispersion within a redshift bin.
+
+    :param z: the redshifts
+    :param dndz: the dndz
+    :param sigma_v: the velocity dispersion within the redshift bin in km/s
+    :return: the shifted and stretched dndz
+    """
+    z = np.asarray(z)
+    dndz = np.asarray(dndz)
+    c = 299792.458  # km/s
+
+    if sigma_v < 0.0:
+        raise ValueError("Stretch Parameter (sigma_v) must be positive")
+    if sigma_v == 0.0:
+        return z, dndz
+
+    n = z.shape[0]
+    dz = z[1] - z[0]
+
+    z_mean = np.trapezoid(dndz * z, z)
+    sigma_s = (1 + z_mean) * sigma_v / c
+
+    z_kernel = (np.arange(n) - n // 2) * dz
+    kernel = np.exp(-0.5 * (z_kernel / sigma_s) ** 2)
+    kernel /= np.trapezoid(kernel, z)
+
+    dndz_new = fftconvolve(dndz, kernel, mode="same")
+    dndz_new /= np.trapezoid(dndz_new, z)
+
+    return z, dndz_new
+
+
+def dndz_stretch_fog_lorentzian(
+    z,
+    dndz,
+    sigma_v: float,
+) -> tuple[npt.NDArray[np.float64], npt.NDArray[np.float64]]:
+    """Apply the Fingers-of-God (FoG) effect on the spec-z distribution.
+
+    This function applies a lorentzian kernel to model the FoG effect
+    due to velocity dispersion within a redshift bin.
+
+    :param z: the redshifts
+    :param dndz: the dndz
+    :param sigma_v: the velocity dispersion within the redshift bin in km/s
+    :return: the shifted and stretched dndz
+    """
+    z = np.asarray(z)
+    dndz = np.asarray(dndz)
+    c = 299792.458  # km/s
+
+    if sigma_v < 0.0:
+        raise ValueError("Stretch Parameter (sigma_v) must be positive")
+    if sigma_v == 0.0:
+        return z, dndz
+
+    n = z.shape[0]
+    dz = z[1] - z[0]
+
+    z_mean = np.trapezoid(dndz * z, z)
+    sigma_s = (1 + z_mean) * sigma_v / c
+
+    z_kernel = (np.arange(n) - n // 2) * dz
+    arg = -np.sqrt(2) * np.abs(z_kernel) / sigma_s
+    arg = np.clip(arg, -700, 0)  # avoid overflow
+    kernel = np.exp(arg)
+    kernel /= np.trapezoid(kernel, z)
+
+    dndz_new = fftconvolve(dndz, kernel, mode="same")
+    dndz_new /= np.trapezoid(dndz_new, z)
+
+    return z, dndz_new
+
+
+class SourceGalaxySpecZStretch(
+    SourceGalaxySystematic[_SourceGalaxyArgsT], Generic[_SourceGalaxyArgsT]
+):
+    """A spec-z convolution for the Fingers-of-God (FoG) effect.
+
+    This systematic convolves the spec-z distribution with a kernel to model
+    the FoG effect.
+
+    The following parameters are special Updatable parameters, which means that
+    they can be updated by the sampler, sacc_tracer is going to be used as a
+    prefix for the parameters:
+
+    :ivar sigma_v: the spec-z stretch.
+    """
+
+    def __init__(self, sacc_tracer: str, kernel: str = "gaussian") -> None:
+        """Create a SpecZShift object, using the specified tracer name.
+
+        :param sacc_tracer: the name of the tracer in the SACC file. This is used
+            as a prefix for its parameters.
+        :param kernel: which kernel to use when convolving the distribution.
+        """
+        super().__init__(sacc_tracer)
+
+        self.sigma_v = parameters.register_new_updatable_parameter(
+            default_value=SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_V
+        )
+
+        if kernel == "lorentzian":
+            self._transform = dndz_stretch_fog_lorentzian
+        else:
+            self._transform = dndz_stretch_fog_gaussian
+
+    def apply(self, _: ModelingTools, tracer_arg: _SourceGalaxyArgsT):
+        """Apply a convolution to the spec-z distribution of a source."""
+        new_z, new_dndz = self._transform(tracer_arg.z, tracer_arg.dndz, self.sigma_v)
+        return replace(tracer_arg, z=new_z, dndz=new_dndz)
+
+
+class SpecZStretch(SourceGalaxySpecZStretch):
+    """Spec-z stretch systematic."""
+
+
+class SpecZStretchFactory(BaseModel):
+    """Factory class for SpecZStretch objects."""
+
+    model_config = ConfigDict(extra="forbid", frozen=True)
+
+    type: Annotated[
+        Literal["SpecZStretchFactory"],
+        Field(description="The type of the systematic."),
+    ] = "SpecZStretchFactory"
+
+    def create(self, bin_name: str) -> SpecZStretch:
+        """Create a SpecZStretch object with the given tracer name."""
+        return SpecZStretch(bin_name)
+
+    def create_global(self) -> SpecZStretch:
+        """Create a SpecZStretch object with the given tracer name."""
+        raise ValueError("SpecZStretch cannot be global.")
+
+
 class SourceGalaxySelectField(
     SourceGalaxySystematic[_SourceGalaxyArgsT], Generic[_SourceGalaxyArgsT]
 ):
diff --git a/firecrown/likelihood/_source.py b/firecrown/likelihood/_source.py
index 4ec27ba36..a7a4a100e 100644
--- a/firecrown/likelihood/_source.py
+++ b/firecrown/likelihood/_source.py
@@ -6,21 +6,27 @@
 from firecrown.likelihood._base import (
     SOURCE_GALAXY_SYSTEMATIC_DEFAULT_DELTA_Z,
     SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_Z,
+    SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_V,
     PhotoZShift,
     PhotoZShiftFactory,
     PhotoZShiftandStretch,
     PhotoZShiftandStretchFactory,
+    SpecZStretch,
+    SpecZStretchFactory,
     Source,
     SourceGalaxy,
     SourceGalaxyArgs,
     SourceGalaxyPhotoZShift,
     SourceGalaxyPhotoZShiftandStretch,
+    SourceGalaxySpecZStretch,
     SourceGalaxySelectField,
     SourceGalaxySystematic,
     SourceSystematic,
     Tracer,
     dndz_shift_and_stretch_active,
     dndz_shift_and_stretch_passive,
+    dndz_stretch_fog_gaussian,
+    dndz_stretch_fog_lorentzian,
 )
 
 # All classes have been moved to _base.py
@@ -34,14 +40,20 @@
     "SourceGalaxySystematic",
     "SourceGalaxyPhotoZShift",
     "SourceGalaxyPhotoZShiftandStretch",
+    "SourceGalaxySpecZStretch",
     "SourceGalaxySelectField",
     "SourceGalaxy",
     "PhotoZShift",
     "PhotoZShiftFactory",
     "PhotoZShiftandStretch",
     "PhotoZShiftandStretchFactory",
+    "SpecZStretch",
+    "SpecZStretchFactory",
     "dndz_shift_and_stretch_active",
     "dndz_shift_and_stretch_passive",
+    "dndz_stretch_fog_gaussian",
+    "dndz_stretch_fog_lorentzian",
     "SOURCE_GALAXY_SYSTEMATIC_DEFAULT_DELTA_Z",
     "SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_Z",
+    "SOURCE_GALAXY_SYSTEMATIC_DEFAULT_SIGMA_V",
 ]
diff --git a/firecrown/likelihood/number_counts/__init__.py b/firecrown/likelihood/number_counts/__init__.py
index 3018a3de5..41b3b15a0 100644
--- a/firecrown/likelihood/number_counts/__init__.py
+++ b/firecrown/likelihood/number_counts/__init__.py
@@ -9,6 +9,13 @@
     MagnificationBiasSystematic,
     PhotoZShift,
     PTNonLinearBiasSystematic,
+    PhotoZShiftandStretch,
+)
+
+# Re-export shared factories from base module
+from firecrown.likelihood._base import (
+    SpecZStretch,
+    SpecZStretchFactory,
 )
 
 __all__ = [
@@ -18,4 +25,7 @@
     "ConstantMagnificationBiasSystematic",
     "PTNonLinearBiasSystematic",
     "MagnificationBiasSystematic",
+    "PhotoZShiftandStretch",
+    "SpecZStretch",
+    "SpecZStretchFactory",
 ]
diff --git a/firecrown/likelihood/number_counts/_factories.py b/firecrown/likelihood/number_counts/_factories.py
index 8492b5415..c8c66b72c 100644
--- a/firecrown/likelihood/number_counts/_factories.py
+++ b/firecrown/likelihood/number_counts/_factories.py
@@ -14,6 +14,7 @@
     MagnificationBiasSystematic,
     PTNonLinearBiasSystematic,
 )
+from firecrown.likelihood._source import SpecZStretchFactory
 from firecrown.likelihood.weak_lensing import (
     PhotoZShiftandStretchFactory,
     PhotoZShiftFactory,
@@ -123,7 +124,8 @@ def create_global(self) -> ConstantMagnificationBiasSystematic:
     | LinearBiasSystematicFactory
     | PTNonLinearBiasSystematicFactory
     | MagnificationBiasSystematicFactory
-    | ConstantMagnificationBiasSystematicFactory,
+    | ConstantMagnificationBiasSystematicFactory
+    | SpecZStretchFactory,
     Field(discriminator="type", union_mode="left_to_right"),
 ]
 
diff --git a/firecrown/likelihood/number_counts/_systematics.py b/firecrown/likelihood/number_counts/_systematics.py
index 5264ae7e9..3fc177210 100644
--- a/firecrown/likelihood/number_counts/_systematics.py
+++ b/firecrown/likelihood/number_counts/_systematics.py
@@ -13,6 +13,7 @@
 from firecrown.likelihood._base import (
     SourceGalaxyPhotoZShift,
     SourceGalaxyPhotoZShiftandStretch,
+    SourceGalaxySpecZStretch,
     SourceGalaxySelectField,
     SourceGalaxySystematic,
 )
@@ -49,6 +50,10 @@ class PhotoZShiftandStretch(SourceGalaxyPhotoZShiftandStretch[NumberCountsArgs])
     """Photo-z shift systematic."""
 
 
+class SpecZStretch(SourceGalaxySpecZStretch[NumberCountsArgs]):
+    """Spec-z stretch systematic."""
+
+
 class SelectField(SourceGalaxySelectField[NumberCountsArgs]):
     """Systematic to select 3D field."""