County Route 521 is a county highway in the U.S. state of New Jersey. The highway extends 43.71 miles from High Street in Hope Township to the New York state line in Montague Township.

In the U.S. state of New Jersey, county routes exist in all 21 counties. They are typically the fourth type of roadway classified below the Interstate Highway, the U.S. Route numbered highway and the state highway. The County Route system is defined by two types in New Jersey. First, 500 Series County Routes, also called state secondary routes, are county highways numbered in a statewide system with three-digit numbers that begin with 5. These roads form a second network of routes that supplement the facilitation of the State Routes. Each 500 Series route is unique and is not permitted to be duplicated in another county for a separate route. The second category is defined as Non-500 Series County Routes. Non-500 Series County Routes include 1-digit, 2-digit, 600 Series, 700 Series and 800 Series. These, by definition, are discontinuous across county borders and must be contained entirely within that county. Unlike 500 Series County routes, these route numbers are unique to each county, and are typically assigned to more local routes than the statewide 500-series county route system. They are typically of a lesser classification of streets like minor arterials or collector roadways rather than major arterials or thoroughfares. In the counties that use 600-series numbers, the selection of this range was coordinated within the state, gradually replacing older systems of mainly one- and two-digit routes.