Diniz jumped for joy after winning in a time of three hours, 40 minutes 37 seconds, the best in Europe this year. Poland's Grzegorz Sudol was second, 1:47 behind and Russian Sergey Bakulin third.
Olympic champion Alex Schwazer, who was second in Tuesday's 20-km walk, pulled out toward the end of the race holding his right hamstring.
The 32-year-old Diniz took the lead on the first lap of the one-km circuit on Barcelona's streets, on a muggy, overcast morning, and remained untroubled by anyone else for the rest of the race.
He blew kisses to the spectators after crossing the line before jumping up and pumping his arms in the air.
"The strategy was to concentrate on myself, not on the others because I had been looking at the others in the past two years and I did not race well," Diniz, who retired after 33 km at the Beijing Olympics and finished a disappointing 12th at the world championships last year, told French TV.
France, which also boasts gold medalists in sprinter Christophe Lemaitre and decathlete Romain Barras, are favorites to take the men's steeplechase title after Olympic runner-up Mahiedine Mekhissi-Benabbad was fastest in qualifying in 8:27.32, while European record holder Bouabdellah Tahri won his heat.
British world champion Jessica Ennis took the early lead in the heptathlon with an assured performance in the 100 hurdles.
She was ahead in her heat at the second barrier and recorded the fastest time of 12.95 for 1,132 points.
Norway's world, Olympic and European champion Andreas Thorkildsen had a scare in javelin qualifying with two no throws before managing 78.82 meters on his final attempt to go through to Saturday's final with the sixth-best distance.
Finland's Tero Pitkamaki, second to Thorkildsen in Gothenburg four years ago, comfortably surpassed the automatic qualifying distance of 81 meters with 83.15 on his second throw.
World high jump champion Blanka Vlasic cleared the qualifying height of 1.92 on her first attempt to book a place in Sunday's final.
Belgium's defending champion Tia Hellebaut, who returned to competition this month after the birth of her daughter in 2009, made the height on her final jump.
Defending champion Ralf Bartels of Germany and Europe's leading shot putter this year, Andrei Mikhnevich, of Belarus, both eased into the final of their event.
A third member of the speedy Borlee family made an appearance at the Olympic stadium when Olivia, elder sister to 400 meter finalists Jonathan and Kevin, finished third in her 200 heat to go through to the semifinals.
Russian Anastasiya Kapachinskaya was quickest in the heats in 23.09.
Norway's under-23 champion Christina Vukicevic was the fastest qualifier for Saturday's 100 hurdles semifinal with 12.83 in her heat.
