Robert Kraft has formally entered a not guilty plea in his prostitution case, according to court records obtained by TMZ Sports.
"The defendant Robert Kraft hereby pleads not guilty to all charges and requests a non-jury trial in the above-styled cause," the court docs state.
As we previously reported, Kraft's attorney accepted the summons from the court on Feb. 26 -- and an arraignment was set for March 27 at 8:30 AM ... the same day Kraft was expected to be at the NFL's Annual League Meeting with other team owners in Phoenix, Arizona.
Kraft had already made a public statement denying he participated in any illegal activity -- in response to allegations he solicited prostitution at the Orchids of Asia Day Spa in Jupiter, Florida.
Officials say they have graphic surveillance video showing the 77-year-old New England Patriots owner entering the spa on January 19 and 20 ... and receiving sex acts in exchange for money.
If convicted on both misdemeanor counts, Kraft faces up to a year in jail.
The NFL is also investigating the alleged incidents -- and Kraft will likely face discipline from the league as well.