Johan Gustav Christoffer Thorsager Kjeldahl (Danish pronunciation: [joˈhæn ˈkʰelˌtɛˀl] 16 August 1849 – 18 July 1900), was a Danish chemist who developed a method for determining the amount of nitrogen in certain organic compounds using a laboratory technique which was named the Kjeldahl method after him.

Kjeldahl found the answer was in developing a technique to determine nitrogen with accuracy but existing methods in analytical chemistry related to proteins and biochemistry at the time were far from accurate.

In order to solve the problem of determining accurate nitrogen content in a sample, Kjeldahl developed a method which involves a two-step reaction: a distillation and a back titration.

He found that ammonium salts can be produced by the reaction between organic compounds and sulfuric acid; this step is a digestion.

During the 1880s, Kjeldahl used potassium sulfate to raise the boiling point of the acid and mercury as a catalyst to speed the decomposition.