The lead singer of The Chainsmokers, Drew Taggart, and his fiancée, Mari Fonseca, got the surprise of their lives over the weekend when they spotted a woman brazenly climbing onto their property and walking right up to the front door ... TMZ has learned.

DT and MF were upstairs in their Los Angeles home Saturday morning when they peered outside and saw an unidentified woman hopping the gate into the front yard, before trying to gain access to their house ... according to law enforcement sources.

Moments later, Drew phoned 911 and officers raced to the scene with lights and sirens on, cuffing the woman in the yard, our sources say.

Sources say the woman told cops she not only lived there ... but she was also DT's wife. This was news to Drew and Mari who, as we reported last month, got engaged.

Our sources say police interviewed Drew and Mari, both of whom said they didn't know the 40-year-old woman, and confirmed she had no business at their house.

Police then spoke with the suspect again -- and determined she needed to be placed on a psychiatric hold.

Prosecutors will review the case for possible trespassing charges.