Sorry to disappoint you just when started to read to this article: there are not beaches in Lisbon (city).
The city is located along the river Tejo, and there are not beaches in the city.

But, there are great news! Close to Lisbon, less than 30min driving time, there are wonderful beaches, where you can spend a great day during your vacation.
Some of these beaches are easily accessible by car, bus or train.

 
Find below an updated list:

Carcavelos

Located less than 30min by train from Lisbon (Cais do Sodré), its one of the most popular beach closest to Lisbon. Avoid weekends during summer period, or arrive early. It gets packed easily. 1mile approximately of beach length.
GoogleMaps: https://goo.gl/maps/j2pS7vPiNmu

Estoril/Cascais

Located approx 45min by train or 30min by car from Lisbon, if you continue ahead after crossing Carcavelos, you will find this amazing area. Considered the Riviera of Portugal, plenty of beaches to spend your day. Additionally, very nice restaurants to visit as well. It’s also a nice place to stay.
GoogleMaps: https://goo.gl/maps/T1e9aHt3Wnq

Costa da Caparica

Less easy to access by public transportation, easier by car, approx 40min from Lisbon (south), its one of the longest extensions of beach in Portugal (approx 30km – 20 miles of beautiful beaches). Spend the day, avoid weekends or get early. There are several beaches all connected to each other, keep on going for a while and choose one in the middle.
GoogleMaps: https://goo.gl/maps/1Xb1dpAxZam

Comporta

If you continue heading South of Lisbon, you will find Comporta. It’s not so close, approximately 1h30m from Lisbon, but it worth the drive. Approximately 45km of beach extension (30miles), mostly desert beaches (specially during week time), it’s a heaven for people who like to be away from the crowds.
GoogleMaps: https://goo.gl/maps/eWUQHGVHQGT2

Algarve

Well, this is definitely not a recommendation to spend the day, because of the distance (6h roundtrip from Lisbon), but definitely a place to consider to spend few days while in Portugal. 150km (120miles) of coastal line, with the most amazing beaches in Portugal. If you don’t like cold water, here you will find the warmest ocean in Portugal.
GoogleMaps: https://goo.gl/maps/j2pS7vPiNmu

Would like to share your thoughts and recommendations ?
Let me know.