Katy Perry is standing up for fellow songstress Beyonce ... saying Jay-Z's wifey did NOT lip-sync.

The "Love On Top" singer was accused of lip-syncing the National Anthem during President Obama's inauguration ceremony in Washington D.C.

We caught John Mayer's ladylove arriving home from her trip to Washington, and when asked her thoughts on Beyonce's performance, Katy told the cameras, "She didn't lip-sync."

Now THAT'S a good friend!

Just to make sure her fans knew that she had rehearsed "The Star Spangled Banner" performance, Beyonce posted the below pics showing off her sheet music. Is this proof enough?