Parnell said Wednesday that he'll run for Pennsylvania's politically divided 17th District, which runs from Pittsburgh's suburbs through Ohio River towns to the Ohio border.
The seat is held by freshman Rep. Conor Lamb, a Democrat who beat third-term Republican Keith Rothfus last year by 12 percentage points.
Parnell made the announcement during an appearance on "Fox & Friends" and on social media, where he posted a photo of himself with Donald Trump Jr.
Parnell is an Army combat veteran who served in Afghanistan and wrote a book about it.
Lamb is a former Marine and federal prosecutor whose grandfather was the state Senate's Democratic floor leader.