M-33 is a north–south state trunkline highway in the US state of Michigan that runs from Interstate 75 (I-75) at Alger in Arenac County north to M-27 near Cheboygan. In between, the trunkline runs through rural sections of the northeastern Lower Peninsula including state and national forest areas. M-33 connects to a handful of parks and crosses several of the rivers in that section of the state. It runs concurrently with three other state highways, sharing pavement to connect through several small communities of Northern Michigan. M-33 was designated by 1919 along a section of the current highway between Mio and Atlanta. The highway also included roadway segments south of Mio that are now parts of other trunklines. The portion south of Mio was rerouted in the mid-1920s, transferring sections to M-72 in the process. The state started extending M-33 in both directions in the 1930s. The current highway segment between Onaway and Cheboygan was the former route of US Highway 23 (US 23) until 1940 when the latter highway was realigned onto an alignment that runs along Lake Huron. Several minor changes have been made to M-33's routing since the 1950s to straighten out curves or finish paving the highway. Route description M-33 starts at exit 202 on I-75 near Alger. From there it crosses a branch of the Lake State Railway and Old 76 Road before turning north. The highway runs north parallel to the Rifle River across the Arenac–Ogemaw County line. It meets the eastern terminus of F-18 and continues through woodland to an intersection with M-55 east of West Branch. North of the junction, the environment transitions to farm land that borders the edge of the Au Sable State Forest to the Rose City area. North of Rose City, M-33 crosses into the Huron National Forest as the highway continues due north into Oscoda County, passing Mack Lake campground. M-33 follows Morenci Avenue into the center of Mio where the highway joins M-72 south of Mio Pond, and they run concurrently together across the Mio Pond section of the Au Sable River out of town. On the north side of the river, F-32 merges in from the east and the three roadway designations run concurrently north. The highway continues to a sweeping 90° turn east near Smith Lake. F-32 separates to turn west and M-33/M-72 turns eastward. The trunkline continues to Fairview where M-33 turns north,