The Klang War or Selangor Civil War was a series of conflicts that lasted from 1866 to 1874 in the Malay state of Selangor in the Malay Peninsula (modern-day Malaysia).

It was initially fought between Raja Abdullah, the administrator of the Klang Valley, and Raja Mahadi. It was joined by Tengku Kudin (Tengku Dhiauddin, also spelt Ziauddin), a Kedahan prince, as well as other Malay and Chinese factions. The war was eventually won by Tengku Kudin and Abdullah's son, Raja Ismail.

In 1854, the sultan of Selangor Sultan Muhammad Shah appointed Raja Abdullah bin Raja Ja'afar as governor of the Klang Valley. Raja Abdullah and his brother Raja Juma'at had previously helped Raja Sulaiman settle a debt incurred during a failed mining venture, and was rewarded with the governorship of the Klang Valley. Raja Mahadi, the grandson of Sultan Muhammad Shah, was the son of Raja Sulaiman who previously served as governor of the Klang Valley; Raja Abdullah's appointment to the post therefore also disinherited Raja Mahdi of the position, which resulted in his resentment.

Raja Abdullah and Raja Juma'at, who had opened very successful tin mines in Lukut (near modern Port Dickson, Negeri Sembilan), obtained funding for the exploration of new tin mines near Kuala Lumpur, at the confluence of the Klang and Gombak rivers, in 1857. The new mines were successful, generating considerable revenue, and the struggle for the control of the revenue, as well as for political power, were essentially the reasons for the war.

Sultan Muhammad died in 1857, and Sultan Abdul Samad succeeded him after a power struggle. Sultan Abdul Samad however only had direct control over the state capital, Langat, and did not have absolute control over the rest of Selangor, which was organized into four riverine provinces – the Bernam Valley, Kuala Selangor, the Klang Valley, and Lukut – which were administered by four autonomous chieftains or governors. When the disgruntled Raja Mahadi initiated the conflict, the Malays were split into two camps in the ensuing war. On Raja Mahadi's side were Raja Mahmud, son of the Panglima Raja of Selangor; Raja Hitam of the Bernam Valley; as well as Sumatran immigrants led by Mohamed Akib and his younger brother Mohamed Tahir (later conferred the title of 'Dato Dagang'). Raja Abdullah's faction included his son, Raja Ismail who continued the war after Raja Abdullah's death, later joined by the Kedahan prince, Tengku Kudin and supported by Sultan Abdul Samad. The Chinese tin miners were also divided between the two camps.

Some of the Malays however switched sides in the course of the war, for example Mohamed Tahir, who helped Raja Mahadi take Klang from Raja Abdullah, eventually switched to Tengku Kudin's side. Syed Mashhor, an Arab-Malay fighter whose father was from Pontianak, was born in Langat, Selangor. He initially supported Raja Abdullah's cause but switched to that of Raja Mahdi. Raja Muda Musa of Kuala Selangor also went over to Raja Mahdi's side.

In the later stages of the conflict Tengku Kudin managed to gain the support of British colonial administrators and in 1873 mercenaries from neighbouring Pahang.

In 1866, Raja Abdullah authorised two traders from the Straits Settlements – William Henry Macleod Read and Tan Kim Ching – to collect taxes from the opium trade in the Klang Valley, which Raja Mahdi was also involved in. Raja Mahdi objected to the tax collection, on the grounds that he should be exempted as he was Selangorean royalty, and refused to pay. Raja Abdullah saw this as an act of defiance by Raja Mahdi towards him. This incident, exacerbated by Raja Mahdi's continued dissatisfaction with being ignored as the successor to Sultan Muhammad for the Selangorean throne following his death in 1857, in favour of Raja Abdul Samad (later became Sultan Abdul Samad), as well as further conflicts between their followers, worsened the already tense relationship between the two princes, which many believe were the initial causes for the outbreak of the Klang War.