The officers were participating in a "bicycle school" on Tuesday afternoon when they rode by the baseball diamond at Naylors Run Park.
There, police say, they caught Jennifer Harvey, 41, of Clifton Heights, and Richard McBride, 37, of North Philadelphia in the act.
Authorities say the two were right on the first base line but, despite being out in the open, no one else was apparently around to see them.
However, the officers had just passed children on a playground and people in tennis courts before making the discovery.
Investigators say they don't know if the two were dating, but don't suspect prostitution.
Both Harvey and McBride were charged with lewdness and disorderly conduct.