Selena Gomez -- New Home Christened with First Trespasser Arrest

Selena Gomez's brand new Calabasas home has lost its cherry ... TMZ has learned a man was arrested on the property last night ... while Selena was home!

As TMZ first reported, Gomez recently purchased a 5-bedroom, 7,200 sq. ft. Calabasas mansion for a cool $3 million.

According to law enforcement sources, Selena and a friend arrived home last night and heard a strange noise. We're told Selena called for her security to check it out.

Our sources say before security could investigate, a guy rang the front doorbell, asking if the pop star was home. The friend answered the door and said she wasn't ... and the guy went on his way.

Law enforcement sources tell us ... sheriff's deputies from the Malibu/Lost Hills department arrived a short time later and found the 21-year-old man had broken into her guest house. He was eventually arrested and charged for burglary and trespassing.