In the last six weeks, Ukrainian crews were trained in the UK to fly and maintain the aircraft - to provide search and rescue capabilities.
Mr Wallace said the UK would also send an additional 10,000 artillery rounds.
He made the announcement from Oslo, where he is meeting allies to discuss ongoing military support for Kyiv.
The defence secretary said the artillery rounds would help the Ukrainian armed forces secure recently-liberated territory.
The Sea King was previously used by both the Royal Air Force and the Royal Navy, with the last craft being retired by the Navy in 2018.
Few countries have sent manned aircraft to Ukraine since the start of the conflict, and requests by its government for Western nations to send fighter jets so far remain unanswered.
At the weekend, Prime Minister Rishi Sunak used a visit to Kyiv to set out a new £50m package of defence aid which included 125 anti-aircraft guns and equipment to counter Iranian-supplied drones.
That followed an announcement earlier this month that an additional 1,000 surface-to-air missiles would be sent.