New Zealand's Rescue Coordination Centre says it has sent a military Orion plane to assist in the search, which is being led by Fiji.
The ferry departed Nonouti Island bound for South Tarawa on Jan. 18. The journey of 240 kilometers (149 miles) was expected to take two days.
Senior Search and Rescue Officer John Ashby said Saturday the MV Butiraoi, a 17.5-metre (57-foot) wooden catamaran, was first reported missing to them late Friday.
Ashby says the vessel underwent repairs to its propeller shaft just before leaving.
Kiribati is a remote, impoverished nation of 33 atolls that is home to about 108,000 people.