Flash Ley School in Stafford was closed in October 2015 when high levels of formaldehyde were discovered.
The colourless gas - found in materials used in plywood, carpeting and foam insulation - has a pungent odour and can cause irritation to the eyes, nose, mouth and throat.
Tests showed the problem occurred after work on the school's floor ducts.
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About 20 pupils at the school reported feeling ill when the smell was first discovered.
Throughout its closure, pupils were based at nearby schools including the Chetwynd Centre, Tillington Manor Primary and Stafford Manor High School.
Staffordshire County Council said remedial work had now finished and continued testing confirmed the air was safe for pupils and staff.
The buildings, on Hawksmoor Road, have undergone a deep clean and the substance believed to be behind the presence of formaldehyde in the air has been removed.
Head teachers Karan Williams and Kerri Fenton said: "This is the news we've all been waiting for.
"Our journey is in its final stages and we are really excited about the new year and continuing our successful future back at Flash Ley.
"We want to express our upmost thanks to everyone; our dedicated staff, the resilient children, loyal parents, the local community and county for supporting us in what has been a unique situation."
A primary school forced to close after a toxic chemical was discovered has reopened.