He had said on Saturday that he and his wife, Nadia-El Nakla, had not been able to contact her parents since the previous day after communications were knocked out and they did not know if they were dead or alive.
Ms El-Nakla’s parents, Elizabeth and Maged, travelled to Gaza from Scotland prior to the conflict to visit family.
On Sunday, Mr Yousaf said on X, formerly Twitter, that he has now heard from them.
He said they are alive but he fears for their safety as they have run out of drinking water.
He said: “We heard from my in-laws in Gaza this morning, they are alive, thank God.
“However, they have run out of clean drinking water.
“The UN resolution must be implemented.
“We need the violence to stop, and for significant amounts of aid to get through without delay.”
The First Minister previously wrote to all political leaders in the UK, urging them to back a ceasefire in Gaza.
In a letter understood to have been sent on Thursday evening, he said Israel has a right to defend itself following the Hamas attacks on October 7, but he added action must be taken now to stop the “staggering humanitarian disaster” unfolding in Gaza before it becomes “cataclysmic”.
A spokesperson for the Foreign, Commonwealth and Development office said: “Our top priority remains the release of hostages and safety of British nationals in Gaza, and for humanitarian aid to get through.
“Humanitarian pauses will be an important operational tool to support these goals. Our support for Israel’s right to defend itself under international law remains unchanged.”