It said a sale-and-purchase agreement was signed on Oct 24.
The value of the transaction was not disclosed.
Medco said other shareholders of the block, Kuwait Energy Co (KEC) and JHL Petroleum Ltd, have also agreed to sell 10.6 percent and 2.1 percent of their respective stakes to Lundin.
Transfer of their participating interests are subject to approval by the government of Cambodia through the Cambodia National Petroleum Authority, it said.
After the transaction, Medco's stake will decrease to 41.3 percent, Lundin will hold 34 percent, KEC with 20.6 percent and JHL with 4.1 percent.
Separately, Medco Energi president Hilmi Panigoro said in a statement that their Libyan unit, Medco International Ventures Ltd, has discovered oil in a third exploration well.
Flow rates were measured through a small, restrictive choke size and "these choke-restricted rates totaled 8,718 barrels of oil per day (gross) from the five intervals tested," he said.
(1 US dollar = 9,099 rupiah)