Mahmoud Nawajaa, coordinator of the Boycott, Divestment and Sanctions (BDS) movement in the occupied Palestinian territories, was arrested at his home in Ramallah on July 30.
Several rights groups, including Amnesty International, had campaigned for his release, including through rallies in Ramallah and Gaza this month.
"Nawajaa was released by an Israeli military court, after 19 days of illegal detention without charges," the BDS movement said in a statement, AFP reported.
Israeli military tribunals exercise jurisdiction over Palestinians in the occupied West Bank.
"Since his arrest on July 30, the Israeli internal security (agency) Shin Bet has failed to present any formal charges against him," the BDS statement added.
The BDS movement calls for a wide-ranging embargo of Israel over its treatment of the Palestinians.
Israel sees BDS as a strategic threat and accuses it of anti-Semitism.