(TMZ) - Khloe wasn't the only theft victim in the Kardashian family -- TMZ has learned, $50,000 in CASH was stolen from Kourtney's house last month ... and cops believe the two heists are related.

Sources tell TMZ, the money was stolen from inside Kourtney's Calabasas home -- following an appearance gig that paid in cash -- hmmmm -- and Kourtney believes it was an inside job.

TMZ reports Kourtney left the money in a safe place in the house for a couple days -- and when she finally went to deposit the loot ... it was gone.

As TMZ reported ... Khloe recently discovered $250,000 worth of jewelry missing from her Tarzana home.

Given the timing and the fact that Kourtney and Khloe work with so many of the same people, TMZ reports police believe the two thefts are connected. 

Cops plan to question people who had access to Kourtney's house around the time of the theft -- and they'll give special attention to people who ALSO had access to Khloe's house.

These chicks need to buy a safe.

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