Per the Mississippi Tax Commission Gambling Winnings website:
Gambling winnings reported on a W-2G, 1099, or other informational return from Mississippi casinos are subject to a three percent (3%) non-refundable income tax. The casinos withhold the tax at the time of payout.
The amount withheld is non-refundable to the taxpayer. Section 27-7-901 of the Mississippi Code provides that the amount of winnings reported on W-2G, 1099 or other informational return from Mississippi casinos are not included in Mississippi income and no income tax credit is allowed for the amount of withholding.
A non-resident taxpayer with only Mississippi gambling winnings and/or losses should not file a Mississippi tax return. The document provided by the casino is considered the income tax return for this type of Mississippi income and therefore is proof that the tax was paid to Mississippi.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.