King Chulalongkorn the Great or Rama V (royal name Phra Chula Chomklao Chaoyuhua) (September 20, 1853 - October 23, 1910) was the fifth king of the Chakri dynasty of Thailand. He is regarded as one of the greatest kings of Thailand.

Chulalongkorn was born September 20, 1853 in Bangkok as the oldest son of King Mongkut (Rama IV) and Queen Debsirinda. His father made sure his son got a very good education, including instruction from European tutors like Anna Leonowens. At age 15 he joined a Buddhist monastery.

His father died when Chulalongkorn was seventeen, and on October 1, 1868, he succeeded him, but the chief minister Chao Praya Srisuriyawongse served as regent for four years, as Chulalongkorn was still too young to rule. During this time he traveled to other countries like Singapore, Java and India, to study western lifestyle and colonialism, which made him the first Thai king to leave the country. Later during his reign he also visited Europe twice, in 1897 and 1907.

On November 16, 1873, he was crowned for the second time. During his travels he had learned about many reforms needed to modernize his country. He managed to keep his country independent even though both France and Britain were colonizing most of Southeast Asia. He did, however, have to cede some territory to these two powers, notably claims on parts of what are now Laos and some northern parts of Malaysia.

King Chulalongkorn modernized the government by introducing a cabinet system, and the semi-feudal provincial administration was changed into a modern administration with provinces (changwat) and districts (amphoe) as it still has today. He declared amnesty for all political prisoners, and also abolished slavery. This liberation is often misundertood but was intended to weaken the power of the Bunnag, a noble family which strictly controlled the Chakri dynasty in those days. Also, this liberation was intended to weaken regional governors to centralize his country. The first railroad of Thailand was opened in his reign in 1896 from Bangkok to Ayutthaya. It was during his reign that the western calendar replaced the traditional lunar calendar. The modern system of coins and banknotes was also introduced by him. He also declared religious freedom, allowing Christianity and Islam to be practiced in the Buddhist country.

The date of his death, October 23, 1910, is commemorated as a national holiday. His second-eldest son, Vajiravudh, succeeded him.

Chulalongkorn University, founded in 1917 as the first university of Thailand, was named in his honor.

In 1997 a memorial pavilion was raised in honor of King Chulalongkorn in Ragunda, Sweden.