On 2 September 2012 Lassad signed for Celtic on a two-year deal, after being released from his contract at Deportivo and passing his medical. His transfer was surrounded by controversy after Levante UD claimed to also have signed the player, who said Celtic was always his first choice.
On 17 November Lassad scored his first goal for his new club, in a 2–0 away win at Aberdeen. He netted his second 11 days later in another away fixture, against Heart of Midlothian.
On 31 August 2013, after featuring in 19 games all competitions comprised, Lassad was released by Celtic.