The explosion was large enough to knock the armoured vehicle carrying the rangers off the road in Saiburi district of Pattani province, a hotspot in an 11-year insurgency that has claimed thousands of lives.
"The bomb was detonated by mobile phone as their vehicle passed by on Monday morning," Colonel Panya Karawanan, commander of Saiburi police, told AFP.
"Two rangers died and five others were wounded," he said.
Shortly afterwards security forces narrowly escaped a second attack, when police were called to the scene of a burning motorcycle a few kilometres (miles) further up the road.
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Thailand annexed the Muslim-majority south more than a century ago, and forced assimilation schemes by the authorities have angered locals in the culturally distinctive deep south.
Resentment at the perceived assault on the region's identity has galvanised support for an insurgency that has left more than 6,400 people dead -- the majority civilians -- since 2004.
Malay-Muslim rebels want greater autonomy from the Thai state, which also stands accused of widespread human rights abuses.
Thailand's junta says it is ready to talk peace with a number of rebel factions, if they reduce the violence.
Rebels conduct near daily ambushes, bombings and executions of officials, security forces or perceived collaborators with the Thai state, such as teachers.
But the majority of the dead are civilians, both Buddhist and Muslim.