Neymar completed his first full training session with Brazil on Sunday.
The Santos forward is expected to feature in Wednesday's Group C encounter against Scotland in Miami Gardens, Florida.
Neymar missed Brazil's first two games of the World Cup, including Friday's 3-0 win over Haiti, with a right calf injury.
The Santos star took to social media to express his delight at training with the rest of the group for the first time since reporting to Brazil's camp last month. Neymar, 34, wrote "Thank you, God. I'm so happy!!!"
Neymar last played for Santos on May 17. His return to fitness is a big boost for Brazil. Neymar is his country's top scorer with 79 goals in 129 international appearances.
"We're all delighted to have him back, back in training and out on the pitch with the rest of us," Flamengo midfielder Lucas Paquetá said on Sunday. "He's a very important player for our national team; he has a wonderful history here and can still help us a great deal. We're happy for him, happy that he's back, and we hope he'll be back on the pitch as soon as possible to help us."
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Neymar is playing in his fourth World Cup and is hoping to lift his first major title with Brazil. Five-time champions Brazil have not lifted the World Cup since 2002.
When asked if he and his teammates are aware that this is Neymar's last chance at World Cup glory, Paquetá said: "I believe so... we players are proud to have the five stars, but also to represent everyone's dream. We want to see that dream come true, just like all of you. And we intend to do our best to make it happen."
Brazil lead Group C, level on four points with Morocco while Scotland sit third with three points.