Observatory Inlet is an inlet on the North Coast of British Columbia. It is a northward extension of Portland Inlet, other sidewaters of which include the Portland Canal. The entrance of Observatory Inlet, from Portland Inlet, lies between Ramsden Point and Nass Point. Ramsden Point also marks, to the west, the entrance of Portland Canal. Observatory Inlet was named by George Vancouver in 1793, because he set up his observatory on the shore of the inlet, at Salmon Cove, in order to calibrate his chronometers. His two vessels, HMS Discovery and HMS Chatham, stayed in Salmon Cove from July 23 to August 17, 1793. During this time a boat surveying expedition under Vancouver himself explored Behm Canal. Vancouver also named three headlands at the entrance of Observatory Inlet: Maskelyne Point, for Nevil Maskelyne, the Astronomer Royal, Wales Point, for William Wales, the mathematical master who sailed with James Cook, and Ramsden Point, after the famed mathematical instrument-maker Jesse Ramsden.
Observatory Inlet has two main arms, the northwest and longer one being named Hastings Arm, fed by the Kshwan River, and Alice Arm, an east arm, fed by the Kitsault River. Hastings Arm is approximately 30 km in length, and runs on a nearly true north-south axis, Alice Arm is approximately 25 km in length and bends sharply, running on a roughly northeast-southwest axis. From their merger to the mouth of Observatory Inlet is approximately 50 km in length, running south-southwest to its merger with the Portland Canal just southwest of Nass Bay, which is a sidewater of Observatory Inlet and the saltwater outer estuary of the Nass.
Of many islands in the inlet, two are located at the intersection of the Hastings and Alice Arms, Granby Island and Brooke Island. Granby Island and Granby Bay are named for the mining company which operated Anyox, and also is the namesake of the Granby River, in the Boundary Country, and the indirect namesake of Granisle, British Columbia, on Babine Lake (Granby Mining Company Ltd.).
At the aperture of Nass Bay is the community of Arrandale on its southern point, with the community of Gingolx (Kincolith) located on its northern shore just inside the bay at the mouth of the Kincolith River. Also entering the Nass estuary just to its east is the Iknouk River.
The smelter ghost town of Anyox, British Columbia is located on the west side of the inlet at Granby Bay, which marks the beginning of Hastings Arm. At the head of Alice Arm is the locality of the same name, which is a former steamer landing and is on the west bank of the mouth of the Kitsault River. 2.5 km due south of it, on the far side of the inlet's shore is the former mining town of Kitsault. Adjacent to the community of Alice Arm, but on the east side of the Kitsault River, is the Nisga'a village site of Gits'oolh (formerly Gitzault Indian Reserve No. 24).