TMZ said the incident happened last Friday night as the couple were leaving the Toscana restaurant in Brentwood, Calif.
Hewitt and Hallisay asked the unnamed photographer to leave them alone, but he ignored them and started taking pictures anyway, the celebrity news website reported.
A scuffle ensued and the photographer said his camera hit him in the face near his eye.
Hewitt felt badly about the incident and let the photographer take a few more pictures, but the paparazzo filed a complaint with police after they left and the matter is now under investigation, TMZ said.
Hewitt got engaged to Hallisay, her co-star on TV's "The Client List," in May and announced she was pregnant with their first child in June.