Þorsteinn is an Old Norse and Icelandic masculine given name. Notable people with the name include:

• Þorsteinn Eiríksson (X century), the youngest son of Erik the Red
• Þorsteinn Erlingsson (1858–1914), Icelandic poet
• Þorsteinn Gylfason (1942–2005), Icelandic philosopher, translator, musician, poet, art enthusiast and intellectual
• Þorsteinn frá Hamri (born 1938), Icelandic writer
• Þorsteinn Gunnarsson (born 1940), Icelandic actor and architect
• Þorsteinn Pálsson (born 1947), Prime Minister of Iceland for the Independence Party from 1987 to 1988
• Þorsteinn Bjarnason (born 1957), Icelandic former professional footballer (goalkeeper)
• Þorsteinn Bachmann (born 1965), Icelandic actor
• Þorsteinn J (full name Þorsteinn Jens Vilhjálmsson), an Icelandic television personality