Jonathan Otley (19 January 1766 — 7 December 1856) was a self-taught English geologist whose detailed observations in the English Lake District set the foundations for the study of that area's geology.

Otley was born on 19 January 1766 at Nook House (also known as The Scroggs) close to Loughrigg Tarn, Cumbria. He came from a working family earning his living making wooden sieves and baskets with his father, over time he learnt how to clean clocks and watches.

He moved from Nook House to Keswick in 1791 and set up in business as a watchmaker, instrument repairer and engraver. He was also a surveyor/map-maker who carried out topographic surveys and the knowledge gained from exploring and observing the Lakeland countryside enabled him to supplement his income by working as a guide. Otley was interested in meteorology and natural history, particularly geology and botany, and one of his "hobbies was planting flowers and ferns beside the mountain springs".

In 1797 Otley moved into a cottage near the centre of Keswick, in King's Head Court, where he spent the rest of his life. The cottage, which still exists, was up a flight of steps and became known as "Jonathan's Up the Steps". Being clever with his hands he soon earned himself the reputation as a chap who could “fettle up maist anything”.

Otley corresponded with several eminent scientists of his day and was visited by many of them, these included William Pearson (who less than 10 years later was instrumental in founding the Astronomical Society of London, now known as the Royal Astronomical Society), in 1809 Otley accompanied Pearson on an excursion up Skiddaw and afterwards Otley was entrusted with the barometer that Pearson had taken with them.

By chance he met John Dalton (who developed the chemical atomic theory) on 6 July 1812 whilst ascending Skiddaw to make meteorological observations. Dalton had been undertaking regular excursions in the Lake District hills for over 15 years and, after a discussion about the barometer he was carrying, Dalton invited Otley to join him the following day on an excursion to make observations on Scafell.

Over the following 25 years Dalton visited the Lake District nearly every summer. He met Otley on at least 19 of those visits and Otley would generally accompany him on his excursions. They investigated Derwent Water's "Floating Island" together and when Dalton was taking barometric readings in the lakeland hills, Otley would check his own altitude measurements against Dalton's barometric measurements. Otley became both an assistant and a friend to Dalton, acting in the capacity of both a colleague and a paid guide.

Otley had been preparing a topographic map of the Lake District which was published in 1818. The map was also engraved by him and an updated copy was included in the guide to the lakes that he first published in 1823. The 1823 guide included his geological insights, Sedgwick described it as "the best guide to the Lakes that ever was written". Later editions, which included material relating to the Yorkshire Dales, provided the first published descriptions of a number of the caves in the Three Counties System of northern England. The guide was widely read by travellers and naturalists of the time, overall eight editions were published between 1823-1849 and a total of over 8000 copies were sold.