Janus (Ianus) Laurentius Jørgensen Ridter (14 August 1854 – 30 November 1921) was a Danish painter and illustrator.

Ridter was born in Aakirkeby on the island of Bornholm as the son of taylor Jørgen Ridder and his wife Cecilie Margrethe Larsdatter.

In 1892, he enrolled at Copenhagen Technical School and also studied privately with the painter Carl Martin Soya-Jensen.

From 1905 Ridter commenced his career as illustrator for C. Ferslew & Co.'s publications (De Ferslewske Blade), especially Aftenposten where he worked from 1906 to 1915.

Together with Johan Thorsø, Ridter created the illustrations of Danish industrial enterprises in Danmarks industrielle Etablissementer which was published in 1887–89.