Gazidis, 52, will join new ECA chairman Andrea Agnelli on the ExCo, and becomes the second representative from the English game, alongside former Manchester United boss David Gill.
Juventus president Agnelli replaces Bayern Munich chairman Karl-Heinz Rummenigge as the ECA's second ever leader.
The ECA is the successor organisation to the G-14 group of Europe's most powerful clubs and now represents the interests of 220 clubs from 53 countries.
In an ECA statement, Agnelli said: "My predecessor, Karl-Heinz Rummenigge, has set a great example and has set the bar of achievements at a very high level.
"I am looking forward, together with the executive board and the administration, to build on his significant results as we manage ECA in the coming years.
"It is a huge honour and a great responsibility the European clubs have entrusted me with. I am looking forward to working closely with all stakeholders within the game in order to protect, promote and develop club football at all levels across Europe."
As well as finding a new leader, the ECA also elected 11 board members and named its four representatives to Uefa's professional football strategy council.
The four representatives are Barcelona's Josep Maria Bartomeu, Ed Woodward of Manchester United, PSG's Nasser Al-Khelaifi and Celtic boss Peter Lawwell.