Charlie Sheen's XXX Ex -- Hornswoggled By Lenny Dykstra ... Allegedly

Convicted felon and legendary scumbag Lenny Dykstra has allegedly struck again ... this time, he's accused of stealing $50k in jewelry from Charlie Sheen's porn star ex-fiancee... TMZ Sports has learned.

The woman pointing the finger is Brett Rossi -- star of the critically acclaimed adult film, "Nice Shoes, Wanna F***?"

Rossi says earlier this year, Dykstra offered to help her sell several pieces of expensive jewelry she was looking to unload ... including 2 Patek Philippe watches, a diamond emerald bracelet and one set of diamond earrings.

For some reason, she trusted the guy ... but quickly had a change of heart when she realized she was TRUSTING LENNY DYKSTRA!!!

Long story short, Brett claims she asked for the stuff back -- but Lenny only returned the earrings, which Rossi claims had been stripped of the REAL diamonds and replaced with FAKES. The ol' switcheroo!!!!!

Rossi claims she called Lenny out for the trickery -- but he responded by making threats against her.

Her lawyer has fired off a letter demanding the immediate return of her jewelry -- but if he's already hocked it, she wants the cash value ... $50,000.

We reached out to Lenny -- so far, no word back.