Hilary Duff Back Together and Making Out with Boyfriend Matthew Koma

Hilary Duff's rekindled an old flame ... and sealed her reconciled relationship with a kiss.

It's back on again for Hilary and Matthew Koma, her music producer boyfriend she split with back in March. The 2 were seen looking lovey-dovey after a coffee run in L.A. Tuesday.

Duff told the ladies of "The Talk" it's the third time she and Koma have dated, adding ... "Timing is such a big deal ... third time's a charm!"

The couple's been dropping hints of their reunion for a few months ... but it looks like it's official now.