"Dismayed by the news arriving from Barcelona. Full support and solidarity with the family and friends of the victims," Ronaldo said in a translated tweet.
Ronaldo is from Portugal, which neighbors Spain. The soccer superstar also plays professionally for Real Madrid in Spain's top pro league, La Liga.
Messi posted about the attack on Instagram ... "I want to send my condolences and all my support to the families and friends of the victims of the terrible attack in our beloved Barcelona, in addition to totally rejecting any act of violence," the FC Barcelona superstar said in a translated message.
"We are not going to give up, we are many more who want to live in a world in peace, without hate and where respect and tolerance are the basis of coexistence."
Former Barca superstar Neymar also chimed in on IG: "May God comfort all families. #PrayForBarcelona. I love you BARCELONA," he said in a translated message.
Rafael Nadal added: "Shattered by what has happened in Barcelona! All my support to affected families and the city," in a translated tweet.
Pau Gasol -- who's from Barcelona -- tweeted, "Sad and horrified by the tragic events that took place today at the #Ramblas of #Barcelona."
"All my support to the victims and their families during these incredibly tough times."