Lisbon's principal flea market has been running since at least the 13th century, now settled at Campo de Santa Clara in the Alfama — a sloping square behind the Panteão Nacional. The name translates loosely as "thieves' market," which gives you some idea of what to expect: second-hand clothing, old vinyl, vintage tiles, antique furniture, books in Portuguese, Soviet-era bric-a-brac, and occasional genuine finds.
The market runs on Tuesdays and Saturdays from around 9am to 6pm. Saturday is larger and busier. Arrive early for the best selection; prices are negotiable, particularly later in the day. The setting — a hilltop square overlooking the river — is good enough on its own terms, and the neighbourhood around Campo de Santa Clara has some excellent cheap restaurants for lunch afterward.