50 Cent Fires Back at Daphne Joy's Rape Claim, Vows to Get Custody of Son

50 Cent is going to war with the mother of his son after she publicly accused him of raping her -- which he says is a totally false claim Daphne Joy is only making in retaliation for him seeking full custody.

Daphne, the mother of 12-year-old Sire, and 50's latest drama is tied directly to one of the lawsuits filed against Diddy. As we reported, Rodney Jones' latest amendment to that suit, claimed Daphne was a "sex worker" for the Bad Boy mogul ... an allegation that immediately caught 50 Cent's attention.

After 50 reacted on Instagram by posting, "I didn't know you was a sex worker, 👀 you little sex worker. LOL 😆" -- Daphne lit him up with a post of her own, saying ... "Let's put the real focus on your true evil actions of raping me and physically abusing me."

Now, 50 tells TMZ ... "The disturbing allegations in the sworn pleadings recently filed in a court case related to Daphne Joy the mother of my twelve-year-old child, has required me to take all necessary legal actions to protect my son Sire."

He continues, "The most recent false and baseless accusations by Daphne Joy are clearly in response to my decision to seek sole custody of my son. My son Sire is my main priority and keeping him in a safe environment is my only focus at this time."

Of course, 50 has been mercilessly trolling Diddy ever since Cassie filed her lawsuit against her ex, and he's continued going in on him this week after the federal raids at Diddy's homes.

It's unclear what Diddy and Daphne's status is right now, but they've been romantically linked in the past ... which is apparently not sitting well with 50.

Thursday afternoon, he went after her again ... posting a pic of her and Diddy with a caption accusing her of getting money from Diddy, and repeating the claim she's a "little sex worker."

For the record, Daphne has said, "The claim that I am a sex worker is 100% false and character assassination. I am retaining an attorney to explore all legal remedies against both Rodney and his attorney."