The decision was formalised yesterday in Brussels when the UK Government reached agreement with the EU Council of Ministers.
Ms Evans worked with the One Wales Government for the move, and negotiations were then taken forward by the member state g
overnment.
At the moment the agreement only applies to the Council but is expected to be extended to other EU institutions.
Jill Evans said: "I'm delighted we've been able to achieve co-official status for Welsh in the EU.
"Whilst initially this will apply to the Council of Ministers, the intention is to extend this so that people and organisations in Wales will be able to correspond and communicate with the EU institutions in either of our two official languages.
"There will also be limited interpretation from Welsh available at some EU meetings.
"It's only right that people in Wales should be able to choose Welsh to deal with the EU institutions if they so wish.
"This is an important day for the Welsh language.
"I want to pay tribute to all those who've taken part in the campaign and I would also like to thank Heritage Minister Rhodri Glyn Thomas and Presiding Officer Dafydd Elis-Thomas for their support."
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