The Ravensbourne rises at Caesar's Well, Keston, four miles south of Bromley town centre, and flows initially in a northerly direction.

The Ravensbourne then flows northwards alongside the A21, passing below Bromley town centre through Church Gardens and Glassmill Reservoir, then on into Beckenham Place Park, the last semi-natural reach of the river.

Further north, it passes through Ladywell Fields, where considerable restoration work has been taking place since 2007/08, with the removal of a long stretch of 1980's concrete channeling, re-routing to more closely match its natural course, and the introduction of terraces and submerged aquatic vegetation (SAV) to provide animal habitat and improve flood control.

South of Bellingham, the small Spring Brook joins the Ravensbourne after flowing only about one mile (1.6 km) from the east through Plaistow and Downham; it crosses the borough boundary from Bromley to Lewisham and follows a narrow strip of parkland named Shaftesbury Park Recreation Ground and Downham Playing Fields along its short course.

The Ravensbourne is also joined by the River Quaggy (known upstream of Sundridge Park as Kyd Brook, and 3.5 miles (5.6 km) in length).

This rises near Princess Royal University Hospital at Locksbottom then flows north through Petts Wood to Sundridge Park in Bromley where its name changes to the River Quaggy.

It then flows northwards through the Mottingham area to Kidbrooke where it then turns westwards through Manor Park in Lee, before joining the Ravensbourne at Loampit Vale in Lewisham.

Just south of the DLR's final northern crossing of the river, on the eastern bank, is Deptford pumping station, constructed in the 1860s as part of the London sewerage system.