Available with Spatial Analyst license.
Summary
Finds the remainder (modulo) of the first raster when divided by the second raster on a cellbycell basis.
Illustration
Usage
The order of inputs is relevant for this tool.

Mod assumes both its inputs are integers. If any inputs are not integer, those inputs will be converted to integers through truncation. Output values are always integers.
Any value modulated (divided) by 0 is assigned NoData on the output. Therefore, any location on the second input that is either 0 or NoData will return NoData for that location on the output.
If the second input value (the divisor) is larger than the first input value (the dividend), the output will be the same value as the first input. For example, if you were to divide a value of 8 by value 10, the integer division calculation will return the input value 8 for the remainder.
In Map Algebra, the equivalent operator symbol for this tool is "%" (link).
See Analysis environments and Spatial Analyst for additional details on the geoprocessing environments that apply to this tool.
Syntax
Mod (in_raster_or_constant1, in_raster_or_constant2)
Parameter  Explanation  Data Type 
in_raster_or_constant1  The numerator input. A number can be used as an input for this parameter, provided a raster is specified for the other parameter. To be able to specify a number for both inputs, the cell size and extent must first be set in the environment.  Raster Layer  Constant 
in_raster_or_constant2  The denominator input. A number can be used as an input for this parameter, provided a raster is specified for the other parameter. To be able to specify a number for both inputs, the cell size and extent must first be set in the environment.  Raster Layer  Constant 
Return Value
Name  Explanation  Data Type 
out_raster  The output raster. The cell values are the remainder of the division of the values in the first input by the second input.  Raster 
Code Sample
Mod example 1 (Python window)
This example returns the value of the remainder (modulus) of dividing the cells in the first raster by the second.
import arcpy
from arcpy import env
from arcpy.sa import *
env.workspace = "C:/sapyexamples/data"
outMod = Mod("degs", "negs")
outMod.save("C:/sapyexamples/output/outmod.tif")
Mod example 2 (standalone script)
This example returns the value of the remainder (modulus) of dividing the cells in the first raster by the second.
# Name: Mod_Ex_02.py
# Description: Finds the remainder of the first raster when divided by
# the second raster on a cellbycell basis
# Requirements: Spatial Analyst Extension
# Import system modules
import arcpy
from arcpy import env
from arcpy.sa import *
# Set environment settings
env.workspace = "C:/sapyexamples/data"
# Set local variables
inRaster1 = "degs"
inRaster2 = "negs"
# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")
# Execute Mod
outMod = Mod(inRaster1, inRaster2)
# Save the output
outMod.save("C:/sapyexamples/output/outmod")
Environments
Licensing Information
 ArcGIS for Desktop Basic: Requires Spatial Analyst
 ArcGIS for Desktop Standard: Requires Spatial Analyst
 ArcGIS for Desktop Advanced: Requires Spatial Analyst