Officers from the two countries swooped on addresses in the early hours of Wednesday in southwestern France and the northern Spanish regions of Navarre and Basque Country, on the orders of Spain's leading investigative judge Baltasar Garzon and a French "anti-terrorist" judge.
A Spanish Interior Ministry official said the raids were aimed at crushing ETA's recruitment, information, and infrastructure capabilities.
Those arrested - 28 in Spain and five in France - were all in their 20s, reported Spain's national news service Efe.
"In many cases these people were recently recruited to ETA, so many of them are very young," claimed a Spanish Interior Ministry official on condition of anonymity.
The Spanish government has cllaimed that a wave of arrests this year has greatly weakened ETA, which is listed as a terrorist organisation by the US and European Union.
About 800 people have lost their lives in the group's 30-year campaign of armed struggle to carve out an independent homeland, in territory straddling northern Spain and southwest France.