Eucalyptus petrensis is a straggly mallee that typically grows to a height of 1.5 to 4 metres (4.9 to 13.1 ft) and forms a lignotuber.

[2][4][5][6] The mallee has a similar appearance to Eucalyptus foecunda but E. petrensis has a more persistent style on the fruit.

[7] Eucalyptus petrensis was first formally described by the botanists Ian Brooker and Stephen Hopper in 1993 in the journal Nuytsia.

[5][8] The specific epithet (petrensis) is a Latin word meaning "found among rocks".

It is restricted to coastal areas between Yalgorup National Park and Dongara.