Rapper Ja Rule will head to prison for two years, after pleading guilty yesterday to 3-year-old gun charges.

The hip-hop star put up a brave front for fans — cheerfully posing and signing autographs — as he left Manhattan Criminal Court, but was clearly unhappy with the looming sentence.

“Today is not a good day for me,” he told reporters. “I’m not in the mood.”

Ja Rule — really Jeffery Atkins — was busted in 2007, when cops searched his Maybach as he left a concert at the Beacon Theatre, and seized a .40-caliber unlicensed handgun inside.

In his plea yesterday, Ja Rule admitted that he had “dominion and control” over the gun. A judge is expected to set a prison date on Feb. 9.