Olof Arenius was a Swedish portrait painter.
Arenius was born in the parish of Bro in Uppland, Sweden. His father was a vicar in Upplands-Bro. After a period of theology studies at Uppsala University, he studied art under David von Krafft (1655–1724). In 1729, he went to the United Kingdom where he was exposed to the art of portrait painters Godfried Kneller (1646–1723) and Mikael Dahl (1659–1743).
Returning to Sweden in 1736, he was named court painter by King Frederick I of Sweden. He died at Stockholm in 1766. His portraits and miniatures in oil are much esteemed, and are to be found in all the public galleries, as well as in the best private collections, in Sweden. Many of them have been engraved.