The Yanchep line, formerly the Joondalup line, is a suburban railway line and service in Perth, Western Australia, linking the city's central business district (CBD) with its northern suburbs. Operated by the Public Transport Authority as part of the Transperth system, the Yanchep line is 54.5 kilometres (33.9 mi) long and has sixteen stations. It commences in a tunnel under the Perth CBD as a through service with the Mandurah line. North from there, the line enters the median strip of the Mitchell Freeway, where nine of the line's stations are. The Yanchep line diverges from the freeway to serve the centre of Joondalup and permanently leaves the freeway north of Butler for the northernmost four stations to Yanchep.

Planning for a rapid transit service to the northern suburbs began in 1987. After several transport modes were considered, including bus rapid transit, an electric railway was chosen. Known during planning and construction as the Northern Suburbs Railway, the project was approved by state cabinet in late 1989 and construction began in November 1989. The line was built under several different contracts, with the total cost of the original project being A$277 million. It used widely spaced stations with bus interchanges and large park-and-rides, distinguishing the line from Perth's three existing rail lines. The line opened on 20 December 1992 as the Joondalup line to limited service and with three stations: Leederville, Edgewater and Joondalup. Four more stations opened in February 1993, and on 21 March 1993, peak service and feeder bus routes commenced. The final station, Currambine, opened on 8 August 1993.

An extension north to Clarkson station and rebuild of Currambine station opened on 4 October 2004, which coincided with the introduction of B-series trains. On 29 January 2005, Greenwood opened as an infill station. The Joondalup line originally through-ran with the Armadale line via Perth station, but in 2005, the line started terminating at Perth station, and on 15 October 2007 the line was rerouted through a new tunnel under the CBD, with two new stations: Perth Underground and Elizabeth Quay. The Mandurah line opened on 23 December 2007 to connect with the southern end of that tunnel. An extension north to Butler station opened on 21 September 2014 and a three station extension north to Yanchep station was opened on 14 July 2024, upon which the line became the Yanchep line.

B-series and C-series trains are the main rolling stock used on the Yanchep line. Trains run at a fifteen minute headway, reducing to as low as a five minute headway in peak, with some services terminating at Whitfords or Clarkson stations during peak. The travel time from Yanchep to Perth Underground is 49 minutes. The Yanchep line received 16,614,973 boardings in the 2024–25 financial year, making it the second busiest line in the Transperth system, after the Mandurah line.

The 1955 Plan for the Metropolitan Region, Perth and Fremantle, also known as the Stephenson–Hepburn Report, proposed a 12-mile (19 km) railway line branching off the Eastern Railway (Fremantle line) at Daglish, then heading west to Reabold Hill and then north to Whitfords Beach via City Beach, Scarborough, and North Beach. The branch was planned to have about eight or nine stations and projected to have about 20,000 daily passenger journeys. The report also proposed a highway to Yanchep, now known as the Mitchell Freeway. When the Metropolitan Region Scheme was adopted in 1963, the land for the proposed highway was reserved but the not the land for the proposed railway.

In 1979, the Fremantle line was closed due to declining patronage, a lack of growth potential, and to save capital and operational costs. A group named the Friends of the Railways was formed to advocate against the closure. Among the group's other proposals was for the electrification of all three lines and the construction of a line to Joondalup. The Transport 2000: A Perth Study report, which was released in 1982, said that the electrification was not economically viable and that buses were sufficient to provide public transport in the northern suburbs. Following a Labor Party victory in the 1983 state election, the Fremantle line was reopened and planning for the electrification of the suburban rail system commenced.

Following the approval of electrification in 1985, attention turned to public transport in the northern suburbs. the Northern Suburbs Rapid Transit Study was commissioned in 1987 by Transperth and the Department of Transport at the request of the Government of Western Australia. The study, published in September 1988 and written by a team of consultants led by Travers Morgan, considered possible modes of transport and the route to take. Routes considered were along West Coast Highway, Marmion Avenue, the western side of the Mitchell Freeway, the median strip of the Mitchell Freeway, the eastern side of the Mitchell Freeway, Wanneroo Road, and Alexander Drive. The route via the Mitchell Freeway's median strip was determined to have the lowest cost and least environmental impact. Transport modes considered were buses on a separate roadway, buses on a guided busway (like the O-Bahn Busway in Adelaide), light rail, heavy rail, automated rubber-tyred trains, high-capacity monorail, and an automated people mover system. A monorail or automated people mover were ruled out due to the cost and unproven nature of those technologies. Light rail was also ruled out due to having a lower operating speed than other modes and the desire to not introduce another mode to Perth's transport system. A bus expressway, guided busway and electric railway along the Mitchell Freeway were considered in further detail.

Public consultation found that the electric railway was the most supported option, but that a majority supported the two bus options combined. The people who preferred rail chose it because of its greater comfort, lower pollution and less crowding. The people who preferred the bus options chose them because there would be no transfer for trips into the Perth central business district (CBD) and the bus options would cost less than a railway. The study suggested that the bus options could be built in multiple stages and the rail option could first be built between Perth and Warwick and later be extended to Joondalup. The two bus options were projected to increase northern suburbs public transport patronage by fourteen percent and the rail option was projected to increase patronage by nine to twelve percent, less than the bus options due to the time taken to transfer from bus to train. The net cost of the railway was estimated to be A$145 million, compared to $87 million for the guided busway and $79 million for the bus expressway. The study concluded that either of the bus options were preferred over the electrified railway, and that further investigation should determine whether to build a bus expressway or a guided busway.