The R21 / P157 is a major north–south provincial route (with a freeway portion designated as a National Road) in eastern Gauteng Province, South Africa. Built in the early 1970s, it remains one of two freeways (the other being the N1) linking Pretoria with Johannesburg, via the R24. As the eastern of the two freeways, it links the Pretoria city centre with OR Tambo International Airport, the N12 freeway, and Boksburg. Between the Solomon Mahlangu Drive on-ramp in Monument Park, Pretoria, and the N12 in Boksburg, the R21 / P157 is an 8 lane highway and motorway (freeway), with 4 lanes in each direction. It has off-ramps leading to Irene, Olifantsfontein, Benoni, and Kempton Park, including a partial offramp to Atlas Road. The route intersects the N1 Highway (Eastern Pretoria Bypass; Danie Joubert Freeway) near Centurion, the R24 near the airport, the N12 and N17 in Boksburg, and the N3 near Vosloorus on the East Rand, where it ends. The section from the N12 to the N3 is not a freeway. As early as the 1970s there were proposals to extend the R21 / P157 freeway further south and the freeway may be linked with the current eastern terminus of the M2 in Germiston in future.

The R21 / P157 is the lowest "R" designated route number in South Africa. However a portion of the R21 / P157 where it exists as a freeway, from Solomon Mahlangu Drive in Pretoria, to the N12 Freeway in the East Rand is both a National Road and a toll road (e-toll highway), having been declared such in 2008, as part of the Gauteng Freeway Improvement Project, when it was also transferred to the South African National Roads Agency. The improvement project increased the number of lanes (previously, the freeway portion had been a dual carriageway freeway, with 2 lanes in each direction, from the Rietfontein Interchange to Pretoria), and installed lighting along the length of the freeway section.