The 'Bad Romance' singer sought medical attention after performing at her 'artRave: The ARTPOP Ball' tour in Denver, Colorado, where altitude sickness is common due to thinner air.
Posting the photo to Instagram, she said: "Altitude Sickness is no Joke! #hitThatHospitalS**t #artRaveDenver many true ravers crowd tonight."
In the photo taken from her hospital bed, a wide-eyed Gaga can be seen with full-on make-up, wearing false eyelashes and thick black eyeliner.
Gaga, who posed with the oxygen mask upside down, is expected to perform at her next concert in Seattle on Friday.
Gaga's hospital selfie comes just hours after she posted a heavy make-up photo on Instagram. Captioning the photo, she said: "Love you little monsters, long and beautiful day. Amazing things ahead."
Touring aside, Gaga is also preparing to release her new album, Cheek to Cheek, with jazz singer Tony Bennett, which is due for release on 23 September.
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Gaga - G.U.Y. - An ARTPOP Film: