It has since been verified as a genuine Renoir by experts at the National Museum in Warsaw.
The picture depicts a sunny scene in the southern French town of Cagnes-sur-Mer, where Renoir spent the last 20 years of his life. Two women dressed in red appear in the centre, accompanied by a little girl.
The Polish authorities are investigating to establish who sent the painting, and the person in the US to whom it was addressed.
Under Polish export laws, valuable art works are subject to a customs duty amounting to 25 per cent of their value when they’re sent overseas.