Phillip Cullen, 57, of Cadbury Heath in Bristol, committed six offences relating to Large Blue butterflies.
Bristol Magistrates Court heard he caught butterflies in Somerset and Gloucestershire and then killed and mounted them for a wall display.
His is believed to be the first prosecution in the UK involving offences related to the species.
Cullen was seen illegally entering and chasing butterflies with a net at Daneway Banks reserve in Gloucestershire on 18 June 2015.
He was also witnessed acting in a similar way at Collard Hill reserve in Somerset between 17 and 20 June the same year.
It was reported to police who searched his home and found up to 30 trays of butterflies.
Cullen had denied catching the Large Blue species found at his home, instead insisting he had purchased them from abroad.
He was convicted of capturing, killing and possessing the insect.
Source: Butterfly Conservation
The Large Blue (Maculinea arion) species became extinct in 1979 but is now thriving after reintroduction in 2004.
In 2004 it was found on nine sites in the country following a major conservation programme.
It has a wingspan of more than two inches and can be identified by a row of black spots on its upper forewing.
A collector has been found guilty of capturing and killing the UK's rarest butterfly.