Alphen aan den Rijn (Dutch pronunciation: [ˈɑlfə(n) aːn dɛn ˈrɛi̯n] (listen), English: Alphen upon Rhine or Alphen on the Rhine) is a town and municipality in the western Netherlands, in the province of South Holland, between Leiden and Utrecht. The town is situated on the banks of the river Oude Rijn (Old Rhine), where the river Gouwe branches off. The municipality had a population of 109,449 in 2017, and covers an area of 132.49 km2 (51.15 sq mi) of which 5.91 km2 (2.28 sq mi) is water.
The municipality of Alphen aan den Rijn also includes the communities of Aarlanderveen, Zwammerdam, and Boskoop. The town is located in what is called the 'Green Heart' of the Netherlands, which is a somewhat less densely populated centre area of the Randstad.
The area around Alphen aan den Rijn has been inhabited for 2000 years. In the Roman era, the Oude Rijn was the main branch of the Rhine River and formed the north border of the Roman Empire. Since the rule of Emperor Claudius (41–54 AD), divisions of the Roman army were stationed here. Consequently, several Roman fortifications were located along the Oude Rijn, including castellum Albanianae in the centre of Alphen. The Romans had also built the first bridge over the Oude Rijn. Alphen was therefore an important commercial site in the area until Germanic raids ended that in 240 AD.
After recurring problems with flooding, especially in Utrecht and Leiden, the Oude Rijn was dammed at Wijk bij Duurstede in 1122, thereby making the Lek River the main branch of the Rhine. The Oude Rijn has not flooded since. During the Middle Ages, Alphen was a fiefdom called Alphen en Rietveld.
In the 17th century, Alphen became prominent again as a hub for commerce. The Oude Rijn was used for boat traffic; there are still portions along the river where the towpath is present.
The current municipality was formed in 1918 through the amalgamation of the smaller municipalities of Alphen, Aarlanderveen, and Oudshoorn. In 1964, the municipality of Zwammerdam was added as well. In 2014 the municipalities of Boskoop and Rijnwoude were amalgamated as well, doubling the land area and increasing the population to over 100,000.
During the Second World War, the majority of Jews from Alphen were deported and subsequently murdered; only a few survived. After the war, the Jewish congregation was disbanded and merged with the one in Leiden. A Jewish cemetery on the Aarkade was founded in 1802, but it was abandoned and razed in the 1960s, with the remains re-interred in Katwijk; in 2012, after fifteen years of community activism by local historian Anke Bakker and CDA council member Alice Besseling, a monument was installed and the area turned into a city park.
Since the 1950's, the city began to grow rapidly. A large new neighbourhood was built on the north side and Alphen became mostly a commuter city. Other urban developments however did create local employment. Since the 1990s, a similar new development was built at the city's south side.
In recent years, a large part of the town centre has undergone a full urban renewal. Many buildings from the 1950s and earlier have been demolished to make place for modern architecture. This "masterplan" included the addition of a new public square next to the riverbank, the construction of a performing arts theatre/cinema, an upgrade of local shops and the creation of pedestrian streets. As of 2006, all of these projects on the left riverbank Hoge Zijde have been finished and a new similar masterplan for the right riverbank Lage Zijde has been developed and is being executed now.
On 3 August 2015, a newly built bridge flap of the under-renovation Juliana Bridge on the Oude Rijn collapsed. It was being hoisted by two cranes, different in size, floating on a pontoon that was not stabilized. It became unbalanced shortly after the lifting started, fell and collapsed onto buildings. There were no injuries and fatalities, many inhabitants having already cleared the area. In total 51 properties were damaged.
- Native white people: 80.7%
- Non-native white people and second-generation descendants: 8.9%
- Turkish people: 1.3%
- Moroccan people: 2.7%
- Surinamese people: 1.8%
- People from the Netherlands Antilles and Aruba: 0.9
- Other non-caucasian non-natives: 3.7% 
Alphen aan den Rijn has the following attractions:
- Avifauna Bird Park – the world's first dedicated bird park, opened in 1950.
- Archeon – a theme park about Dutch History, with 43 replica buildings from prehistoric, Roman, and Medieval eras. It opened in 1994.
- Zegersloot Recreation Park – park with an artificial lake, popular for hiking, cycling, windsurfing, wakeboarding and water skiing.
- 20 van Alphen – International 20 kilometres run. Held every year (since 1952) on the second Sunday of March. The 2006 event was marked by a world record when Haile Gebrselassie posted a time of 1 hour 11 minutes 37 seconds for 25 km. This was his 22nd world record breaking performance.
- Year market – third Wednesday in September. Regional products.
- Lakeside Festival – end of August. Music festival near Zegersloot.
- Old timer day – second Saturday of September. Old cars, tractors, steam engines.
- LAURA – four-day bicycle event Leiden, Amsterdam, Utrecht, Rotterdam and Alphen aan den Rijn which is the start and finish place, is held the first week of July.
- Tean International – Challenger level ATP Tour tennis tournament, played at the Alphense Tennis Club, from 30 August to 8 September 2008.
- Midwinter Fair and Midzomer Fair – Fantasy festivals at the Archeon.
- Vakantiespel – One of the largest events for children between 6 and 12 year old in the Netherlands. Held every year in August.
The town is served by Alphen aan den Rijn railway station. It is just south east of the town centre. The bus station is located near the railway station.
In total, there are 11 parties in the city council that consists of 35 councillors. Liesbeth Spies of the CDA is the current acting mayor. The council of mayor and alderman run the city on a day-to-day basis and report to the city council.
Current (2016) aldermen in Alphen aan den Rijn are:
- M.H. du Chatinier (ChristenUnie)
- C.J. van Velzen (CDA)
- G. van As (Nieuw Elan)
- T. Hoekstra (VVD)
- H. de Jager (CDA)
- E. de Leest (D66 - since 21-04-2016 instead of W.J. Stegeman)
The information and publishing company Wolters Kluwer is based in Alphen aan den Rijn.
- J.C. Bloem (1887–1966), poet
- Petrus Wernink (1895–1971), sailor
- Matthijs van Heijningen (born 1944), film producer
- Wouter van Pelt (born 1968), hockey player
- Arjan van Heusden (born 1972), footballer
- Evert-Jan 't Hoen (born 1975), baseball player
- Wilbert Pennings (born 1975), high jumper
- Jeannette Pennings (born 1977), athlete
- Martin Verkerk (born 1978), tennis player
- Salah Edin (born 1981), rapper
- John Heitinga (born 1983), footballer
- Yoep Frijling (AKA Biermondje) (born 1993), Comedian.
- Mike Griffioen (born 1995 ), Singer