Israel and Egypt largely sealed Gaza after Hamas, a Palestinian faction, seized the coastal territory from rivals Fatah last year.
The Free Gaza Movement has been sailing ships to Gaza for several months, highlighting the plight of the residents and bringing in urgently needed medical supplies. They also succeeded in shipping out five Gaza students in a previous trip.
Israel has warned the activists not to enter the closed military zone it maintains around the Gaza Strip but has not interfered with any of the voyages.
Israel allowed the boat carrying pro-Palestinian activists to sail to the Gaza Strip in the fourth such voyage.
The mission was intended to highlight the conditions of Gaza's 1.5 million residents, who are running short of basic supplies and power.
Mike Cushman, a professor at the London School of Economics, said: "We came here to help some students to get out so they can continue their studying. We will bring some back with us."