Member of the European Parliament, 2014–2017

In the 2014 European elections, De Masi became a Member of the European Parliament. During his time in office, he served on the Committee on Economic and Monetary Affairs and the Special Committee on Tax Rulings and Other Measures Similar in Nature or Effect (TAXE 2).^[2][3] In addition to his committee assignments, he was part of the Parliament's delegation for relations with South Africa.

Member of the German Parliament, 2017–2021

De Masi was a member of the German Bundestag since the 2017 elections, representing Hamburg. In parliament, he was serving on the Finance Committee. In addition to his committee assignments, he was part of the German-British Parliamentary Friendship Group, the German-Italian Parliamentary Friendship Group, and the Parliamentary Friendship Group for Relations with the Southern African States. Since 2019, he was also a member of the German delegation to the Franco-German Parliamentary Assembly. In March 2021 De Masi said that he will not run for the Bundestag in the 2021 German federal election.^[4]

Di Masi has long been a companion of the political figure Sahra Wagenknecht. He founded the “Get Up” ("Aufstehen") movement together with Wagenknecht in 2018.^[5]

On 13 September 2022, De Masi announced that he left The Left Party, stating that he would not want anymore to be taken into responsibility for the blatant failure of the relevant actors in this party.“^[6][7]

Engagement in BSW

He joined the new populist party of his former boss Sahra Wagenknecht. De Masi appeared at the newly formed party's press conference on January 8, 2024, and indicated that he was seeking political office with the BSW at 2024 European Parliament election.^[5][8] Together with Thomas Geisel, he wants to become the party's top candidate.^[9]