Krishnacharya Purnaiah (1746 – 27 March 1812), popularly known as Dewan Purnaiah, was an Indian administrator, statesman, and military strategist who served as the first dewan of Mysore from 1782 to 1811. He was instrumental in the restoration of the rule of the Kingdom of Mysore to the Wadiyar dynasty. After the death of Tipu, he continued to advice Lakshmi Devi, the queen regent to the newly installed monarch Krishnaraja Wodeyar III.

Purnaiah was known for his accounting skills, prodigious memory, and proficiency in several languages. He was also a wartime military commander while serving under Tipu. After Tipu's defeat, he served as the dewan to Krishnaraja Wodeyar III. Wodeyar was educated and mentored by Purnaiah in the latter's early years .

Krishnacharya Purniah was born in 1746. He was fluent in Kannada, Marathi, Sanskrit, and Persian. He understood English but could not read or write the language.^{[citation needed]}

At the age of eleven, Purnaiah lost his father and had to seek employment to support his family. He started writing accounts at a trader's shop. This grocer had close contact with a rich merchant, Annadana Shetty, who supplied large quantities of groceries to Hyder Ali's palace and army.

Purniah earned the confidence of Hyder Ali thanks to his aptitude in accounting and excellent handwriting. Endowed with prodigious memory, proficiency in multiple languages, and sheer hard work, Purniah became the head of the accounting department, and minister, and a confidant and close advisor of the ruler. Hyder Ali also granted Purnaiah a jagir in Maralahalli, Mysore. From then on, many of the ruler’s decisions, including political matters, were shaped by Purnaiah’s counsel.

When Hyder Ali died near Chittoor in 1782, his son Tipu was encamping in the Malabar coast. Purniah kept Ali's death a top secret and sent the word only to Tipu by the speediest way possible. Meanwhile, Ali's body was kept embalmed, and business went on as usual. Purniah thus played a key role in keeping the news of Ali's death confidential owing to adversaries who could have seized this advantage and tried to usurp power. Purniah thus paved the way for the succession of Tipu. He soon became a member of Tipu's inner cabinet, eventually being labelled Dewan for the first time.

After a series of wars with the British East India Company, Tipu was defeated in the fourth one and was killed in 1799. Soon, with lobbying efforts from Maharani Lakshmi Devi, Purniah met with George Harris, the commander-in-chief of the Madras Army, for the handing-over of the kingdom's government to the deposed Wadiyars. A subsidiary alliance was struck, and the maharani became the Queen Regent of Mysore for the infant prince Krishnaraja Wodeyar III. Purniah became the regent's adviser and educated and trained the young prince. On 27 December 1807, in recognition of his services, the queen regent awarded Purnaiah the feudal estate of Yelandur.

Krishnaraja Wodeyar III attained the age of 16 in early 1810, reaching the recognised age of discretion. Consequently, the British Resident of Mysore Arthur Cole invested the kingship in the prince in 1811. Purnaiah continued to advise the new maharaja for a brief period before retiring that year. The presiding Resident John Malcolm honoured him on his retirement by presenting him a horse, an elephant, and a rich killat. He was also honoured with handsome grants and a large pension for his services.