Using numerical integration, in 1969 Penston (Mon Not R Astr Soc 144:425–448, 1969) and Larson (Mon Not R Astr Soc 145:271–295, 1969) independently discovered a self-similar solution describing the collapse of a self-gravitating asymptotically flat fluid with the isothermal equation of state $$p=k\varrho $$
p
=
k
ϱ
, $$k>0$$
k
>
0
, and subject to Newtonian gravity. We rigorously prove the existence of such a Larson–Penston solution.