Arthur C. Clarke but the idea was older.
Here's a quote from the Wikipedia page.
In the 1870s Nikola Tesla, while recovering from malaria, conceived a number of inventions including a ring around the equator, although he did not include detailed calculations. As recounted in his autobiography My Inventions (1919):
Another one of my projects was to construct a ring around the equator which would, of course, float freely and could be arrested in its spinning motion by reactionary forces, thus enabling travel at a rate of about one thousand miles an hour, impracticable by rail. The reader will smile. The plan was difficult of execution, I will admit, but not nearly so bad as that of a well-known New York professor, who wanted to pump the air from the torrid to the temperate zones, entirely forgetful of the fact that the Lord had provided a gigantic machine for this very purpose.
Arthur C. Clarke's novel The Fountains of Paradise (1979) is about space elevators, but an appendix mentions the idea of launching objects off the Earth using a structure based on mass drivers. The idea apparently did not work, but this inspired further research.
Paul Birch published a series of three articles in the Journal of the British Interplanetary Society in 1982. Anatoly E. Yunitskiy, author of string transport idea, also published a similar idea in USSR in 1982 and later explored it in detail in his book published in 1995.
Andrew Meulenberg and his students, from 2008 to 2011, presented and published a number of papers based on types and applications of low-Earth-orbital rings as man's "stepping-stones-to-space". An overview  mentions four applications of orbital rings.
- Orbital ring - Wikipedia
So, the idea was a fantasy back before Arthur C. Clarke but he was the first to use it in a story.