The First River, in the state of New Jersey in the United States, is the first main tributary of the Passaic River encountered while travelling upstream from its mouth at Newark Bay. However, in the 19th century, it was completely paved over, and now flows underground through sewers until discharging into the Passaic. It flowed through, and now flows under, what is now Branch Brook Park in the city of Newark.

It was also known as Mill Brook, having supported several mills, including grist mills for the earliest settlers of Newark in the 17th century.

It began to be covered in 1863, and the covering was complete by 1890. Today, it flows through two sewer tubes, each six feet nine inches high by nine feet three inches wide.

Tributaries

(As encountered travelling upstream)

(None)