Considering that biplanes were made obsolete a long time ago due to the advent of efficient monoplanes, and made totally obsolete when the jet engine became popular, but I've been wondering, when in history was the last known usage of biplanes in battle? Whether it being in a combat or recon role?
1991, former Yugoslavia, that I am aware of. However, since the An-2 Colt is still a service aircraft (or its Chinese derivative), I am sure that there is another combat op or two left in it. The Colt is a very large biplane used primarily for troop transport, battlefield insertion, or aerial resupply, and has hung around because of its incredible ability to fly low and slow. These days a good helicopter will do the job as well or better, but the Colt makes a very good alternative while being a whole hell of a lot cheaper. That said, in 1991 it was used as an ad-hoc bomber.
The last combat-roled biplane I can think of is the Po-2, used as a light bomber in the Korean War. Credited with a kill on an F94 Starfire jet which slowed down so much to engage it that it stalled and fell out of the sky. In the West, my money is on the Fairey Swordfish, which was out of service in 1945.
As noted below biplanes are actually still likely to be encountered in war for some time to come, thanks to the An-2.
To put aside odd cases, night harassment, one-offs and times when biplanes were dug out of the warehouses and training schools in desperation, here are a few cases of the use of the last fighter biplanes in service.
The last claim by a Gloster Gladiator serving in a fighter unit was made in East Africa on October 24, 1941, the victory being over an Italian CR-42, another very late fighter biplane. These two aircraft had engaged in a lot of combat in the previous 18 months or so in Africa, Greece and the Mediterranean.
In the USSR, the Baltic Air Fleet was among the units using the I-153, a biplane fighter with a retractable undercarriage, into 1942. They were used for ground attack for some months after this, air-to-air combat duties being taken over by the faster, monoplane I-16 and then, as combat attrition and wastage destroyed these obsolete machines, by modern monoplanes. In July 1942 I-153 pilots made claims against German bombers attacking Kronstadt naval base near Leningrad. As in many air forces, biplane fighters ended their days as point-defence interceptors.
Interestingly, it was the Soviets who pushed biplane technology furthest, putting ever-larger engines into the I-153 airframe, culminating in the I-190, until the exercise was clearly futile.