Kylie Jenner and Travis Scott kept it low-key for her 20th birthday ... maybe they're saving the bang for her 21st.

Kylie and Travis hit up Cinepolis Theater in L.A. on Thursday night. Unclear why Kylie's hiding her face ... she's been plastering her mug and curves all over Instagram leading up to and on her birthday. Not that we're complaining.

Khloe and her NBA superstar bf, Tristan Thompson, also tagged along for movie night. Not sure what flick they checked out ... but couldn't have been better than this chick.