Horst Söhnlein (born 13 October 1943) is a German activist and former member of the Baader-Meinhof Group who was convicted of arson in 1968.
On 2 April 1968, along with Andreas Baader, Gudrun Ensslin and Thorwald Proll, he set fire to two department stores in Frankfurt as a protest against "the genocide in Vietnam". All four were arrested two days later.
All four of the defendants were convicted of arson and endangering human life and were sentenced to three years in prison. In June 1969 they were temporarily paroled under an amnesty for political prisoners, but in November of that year, the Federal Constitutional Court (Bundesverfassungsgericht) ordered that they return to custody. Unlike his three accomplices, who fled to Paris, Horst Söhnlein complied with the order.