The Spaniard joined the Gunners from Malaga in January 2013.
Two months later, he netted in a Premier League meeting with Swansea.
He had failed to add to that effort in English top-flight competition until opening the scoring against the Seagulls.
1660 - Nacho Monreal has scored his first Premier League goal since 16th March 2013 vs Swansea; 1660 days ago. Cheesy. pic.twitter.com/n4yWPlp1Gh— OptaJoe (@OptaJoe) October 1, 2017
Monreal is hardly renowned for his prowess in front of goal, with his strengths very much lying in the defensive third of the field.
He is not a buccaneering full-back, and has filled in at centre-half for the Gunners at times.
The 31-year-old now has four goals to his name, however, as he slotted home after Hector Bellerin's shot was blocked.
Aside from his effort against Swansea, his other two strikes came in the FA Cup – in victories over Manchester rivals United and City.
All four of his goals have come at different venues - the Liberty Stadium, Old Trafford, Wembley and now Emirates Stadium.