EXCLUSIVE

Robert Herjavec might want to invest in something that reminds him to turn on his home security system, because he didn't ... and it cost him big time.

The "Shark Tank" star's Hollywood Hills house was burglarized ... according to law enforcement sources. We're told a thief or thieves hit his pad last weekend, busting in through a window and making off with more than $100k worth of jewelry, cash and some other items.

Our sources say Herjavec's security alarm was not activated at the time of the break-in, so cops weren't called to his place until he got home and discovered it. We're told it's unclear if this heist is connected to other burglaries in the area.

Shame, Robert and the rest of his 'Shark' pals didn't strike a deal with the inventor of Ring (which used to be called DoorBot) when he was on the show. One of those would've come in handy.