Sir Michael Fallon said the government would make a "bigger commitment to peacekeeping, particularly in Africa".
The UK needed to help states "on the brink of failure", or face instability that leads to extremism, he added.
More than 80 nations will take part in a London peacekeeping conference later.
Last year David Cameron said the UK would send about 70 troops to Somalia and up to 300 to South Sudan.
BBC defence correspondent Jonathan Beale said only a handful of the troops for South Sudan had arrived so far, but Sir Michael's announcement means the total there could reach 400.
Sir Michael told BBC Radio 4's Today programme: "If you don't help states that are on the brink of failure, you get the instability that leads to extremism, war lords and eventually to terrorist groups and then you end up with mass migration.
"So you have to deal with the problem further back, and that's why we've decided now to make a bigger commitment to peacekeeping, particularly in Africa."
One of the first tasks for the British troops in South Sudan will be to build a field hospital.
An extra 100 military personnel will be sent to South Sudan as the UK looks to "step up" its United Nations peacekeeping commitments, the defence secretary has said.