Three young men were detained early Sunday morning after being spotted in a small dinghy inside the harbour.
The three, believed to be Moroccans aged between 18 and 23, claimed to have paddled across the Strait of Gibraltar from Morocco trying to reach Spain.
They were seen in the harbour basin to the rear of Europlaza by HM Customs officers and were subsequently arrested by the Royal Gibraltar Police.
All three are in good health and are currently being processed at New Mole House police station.
They are being held on suspicion of being in Gibraltar without a valid permit.
The Strait of Gibraltar, the narrowest crossing between Europe and Africa, is a key hotspot for clandestine migration. Every year, hundreds of migrants attempt the treacherous crossing in search of a better life.
But most aim to reach the Spanish coastline and it is rare for them to land in Gibraltar.
Even so, this is the second time within the space of a month that migrants trying to enter Spain have ended up in Gibraltar.
Last month, nine men were detained after crossing the strait in a small zodiac and making landfall near Catalan Bay.