"The Egyptian navy arrested nine fishermen while they were out fishing on the line between Gaza and Egypt," union official Nizar Ayesh told AFP.
They were taken ashore in Egypt and detained, he added.
A similar incident in 2013 saw five Gazan fishermen jailed for a year.
Relations have deteriorated between Egypt and Hamas, which controls the Palestinian coastal enclave, since the Egyptian army ousted Islamist president Mohamed Morsi in July that year.
And an Egyptian court declared Hamas's armed wing a terrorist group in January.
Cairo has kept the border crossing with Gaza, blockaded by Israel, almost completely shut since October, when a bombing in Egypt's Sinai Peninsula killed 30 soldiers.
Under the Israeli blockade, fishermen are allowed to trawl the waters up to six nautical miles offshore, but have complained of being fired at by the navy before they reach that limit.
A fisherman was shot dead by Israeli forces earlier this month.
Egypt accuses Hamas of supporting jihadist militants in Sinai.
The army has also created a wide buffer zone along the Gaza border aimed at preventing infiltration by militants.
In January, Egyptian border guards shot at Gazans it said had illegally crossed the border, killing a 17-year-old boy.