the discovery of marginal utility is always attributed to the Prussian Hermann Heinrich Gossen.

Gossen's "Three Laws" (1854) can be stated as follows (HET):

(1) the amount of utility derived from the consumption of a good declines with each additional unit of that commodity (i.e. diminishing marginal utility, or, to use Gossen's term, "diminishing worth of the last atom".)

(2) a person maximizes his utility when he distributes his income among various goods so that he obtains the same amount of satisfaction from the last unit of each good or, if money is being used, he obtains the same amount of satisfaction from the last unit of money spent upon each commodity (i.e. equality of the ratio of marginal utilities to the ratio of prices, i.e. MU_i/p_i = MU_j/p_j for any two goods i, j).

(3) a good has value only when the demand for it exceeds supply (i.e. subjective scarcity is source of value).

Wikipedia notes: In the 1870s, Leon Walras, Carl Menger, and William Stanley Jevons each reintroduced the theory of marginal utility. During discussions of which of those three had been the first to formulate the theory, a colleague of Jevons discovered a copy of Die Entwicktlung*. Gossens was recognised as the original author, and his work was reformulated in a less mathematical way, to make it more intelligble to the public.

From the History of Economic Thought Website: The concept of diminishing marginal utility -- i.e. that equal increments of a good yield diminishing increments of utility -- was already widely known. Daniel Bernoulli (1738) had employed this concept to solve the St. Petersburg Paradox. The utilitarian Jeremy Bentham (1789, 1802) had certainly stated the idea. Lloyd (1833), Senior (1836), Jennings (1855) and Hearn (1864) were well aware of diminishing marginal utility as well. The question was one of connecting it to demand, which these writers failed to do clearly.

Well, and I better don't start a discussion on the History of Calculus ;-D.

*Die Entwicklung der Gesetze des menschlichen Verkehrs und der daraus fließenden Regeln für menschliches Handeln (The development of the laws of human intercourse and the rules of human action).