The 1524mm-gauge 600V dc vehicles will be used on Line 4, which links Riga Central Market with Imanta.
The order comprises 15 three-section vehicles and five four-section LRVs. All 20 vehicles are due to be delivered next year.
Tendering for the contract began last September and Stadler Poland and Pesa also submitted bids.
In May 2008 RS awarded Škoda a contract to supply 20 three-section 15 T ForCity low-floor LRVs, which entered service in 2010. A further six four-section vehicles were delivered in 2012. RS uses its existing ForCity fleet on lines 6 and 11.