Jackson Bay / Okahu is a gently curving bay 24 kilometres wide, located on the West Coast of New Zealand's South Island. It faces the Tasman Sea to the north, and is backed by the Southern Alps. The westernmost point of the bay is marked by the headland of Jackson Head; in the northeast the end of the bay is less well defined, but the small alluvial fan of the Turnbull and Okuru Rivers might be considered its farthest point. The small Open Bay Islands lie five kilometres off the coast at this point.
The bay marks a major change in the terrain of the west coast. To the north, narrow fertile plains lie between the mountains and the sea, allowing for moderately intensive farming of livestock. To the south, the coastal plains disappear as the land becomes steeper and more majestically mountainous. Within 60 kilometres, the first of the deep glacial valleys that further south become the fjords of Fiordland start to become evident, with Lake McKerrow at the foot of the Hollyford Track.
The bay marks the farthest southern extent of the West Coast's road network: the small road which meanders along the coast from Haast, 32 kilometres to the northeast, terminates at the sleepy fishing village of Jackson Bay, close to Jackson Head. This was the site of the landing of the early settlers of the area, and is close to the mouth of the Arawhata (or Arawata) River. The third river to enter the Tasman along this stretch of coast is the Waiatoto River, which enters the bay 10 kilometres to the east of Jackson Head.
A renowned whaling station was established here to target migrating whales such as southern right and humpback whales.