A penguin that escaped from a Tokyo aquarium remains on the lam and was last seen swimming in a nearby river, according to a report.

The Humboldt penguin, one of 135 at Tokyo Sea Life Park, was first spotted on Saturday at the mouth of the Old Edogawa River, which runs into Tokyo Bay, the Japan Times reported.

On Sunday, at 11 a.m. local time, the head of a nearby bird park snapped a photo of the runaway penguin and alerted officials at the aquarium. At 12:30 p.m. a visitor took some photos of the swimming penguin.

"Since then, we don't have any specific information about the penguin's whereabouts," park official Takashi Sugino told the Japan Times.

The 60 to 70-cm-tall penguin would have had to get past a two-metre high net, he said.

There was no way for the year-old penguin to get over the net, but there are gaps which cats and frogs often pass through, Sugino added.

"The penguin may have slipped through under the net and escaped," he said.

Officials have put up posters inside the park and posted a notice on its website seeking tips from the public.