El radio solar (R☉) es una unidad de longitud empleada en astronomía y astrofísica para medir comparativamente el radio de las estrellas y de otros objetos astronómicos de grandes dimensiones. Al igual que en el caso de la masa solar, se escoge el Sol como referencia por ser la estrella más cercana a la Tierra. Por tanto, una unidad de radio solar es igual al radio del Sol, que equivale a unos 109 radios terrestres.

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Solar radius is a unit of distance used to express the size of stars in astronomy. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: The solar radius is approximately 695,700 kilometres (432,288 miles), which is about 10 times the average radius of Jupiter, 110 times the radius of the Earth, and 1/215th of an astronomical unit, the distance of the Earth from the Sun. It varies slightly from pole to equator due to its rotation, which induces an oblateness in the order of 10 parts per million. (See 1 gigametre for similar distances.) The unmanned SOHO spacecraft was used to measure the radius of the Sun by timing transits of Mercury across the surface during 2003 and 2006. The result was a measured radius of 696,342 ± 65 kilometres (432,687 ± 40 miles). Haberreiter, Schmutz & Kosovichev (2008) determined the radius corresponding to the solar photosphere to be 695,660 ± 140 kilometres (432,263 ± 87 miles). This new value is consistent with helioseismic estimates; the same study showed that previous estimates using inflection point methods had been overestimated by approximately 300 km.

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