Russian president Vladimir Putin said the deal brought “a sigh of relief” to the world.
In a statement published on the Kremlin website, Mr Putin also said that “Russia will do its utmost to make sure that the Vienna agreement is fully implemented, thus contributing to the international and regional security.”
Syrian president Bashar Assad said the deal is “a historic turning point” in the history of Iran and its relationship with countries of the region and beyond.
Mr Assad’s comments came as he congratulated Iranian president Hassan Rouhani on the agreement.
Mr Assad said coming days will witness a “strengthening of the constructive role played by Iran in supporting the rights of nations”.
Iran is a key supporter of Mr Assad. It has poured in millions of dollars’ worth of help to prop up his army in recent years as it fights an insurgency and rebels seeking to topple his rule.
French president Francois Hollande called on Iran for help in the Syrian conflict during a Bastille Day speech in which he praised the landmark agreement.
Mr Hollande said the deal shows the “world is moving forward” and that “Iran must show that it is ready to help us end the (Syrian) conflict.”
But Mr Hollande also said that world powers must carefully watch how Iran uses the billions of dollars in relief from international sanctions it gets in exchange for curbing its nuclear programme.
“Now Iran has a bigger financial capacity, we need to be extremely vigilant on what Iran will be,” he added in the televised speech.
Meanwhile, French foreign minister Laurent Fabius told newspaper Le Monde that the Iran deal was “sufficiently robust” to last 10 years.