The Atletico Madrid centre-back had previously hinted at committing himself to Spain in international football, saying the country "has given me everything".
As such, his decision to accept a call-up from Didier Deschamps and France was met with some criticism in Spain, with Lucas making his France debut in Friday's 3-2 defeat by Colombia.
Despite the flak, Lucas is adamant he only ever wanted to play for the country of his birth.
"I know people talked about it a lot but my choice is final, and it's France," Lucas told reporters on Sunday.
"I'm proud to be in blue, to wear this shirt and as long as I'm on the pitch I'll make sure I do everything in order to defend this shirt to the death.
"Certainly, when you're far from France you don't know much [about the national team], so you start to wonder.
"But as soon as I got the call from the coach, who was urged to call me by Antoine Griezmann, and he told me he wants me, I didn't hesitate for another minute. Not even for 30 seconds. I said yes straight away.
"I understand that some people will see this negatively, because the French don't know me like the Spanish do.
"Just so you know, I'm 22 now and I've lived in Spain for 18 years. So, when I say that Spain gave me everything, personally and professionally, it's the truth and I can't deny that.
"I studied there, my whole football career was there. And when I say I speak better Spanish than French, that is also true. I won't hide it.
"When you live abroad and don't speak your own language much, you lose the accent a bit, as you can see.
"So yes, Spain gave me everything, but it doesn't change the fact that my home country is France and I am proud to defend the blue jersey."