Nadal, who overtook Roger Federer at the top of the men's rankings shortly after beating the Swiss star in the Wimbledon final, is the first Spanish man to receive the honor.
"It has been an incredible year for me and I will never forget all that's happened,'' said Nadal, who also won his fourth straight French Open title in 2008, beating Federer in the final at Roland Garros for the third year in a row. "To finish the year ranked No. 1 was really special.''
The 22-year-old also won the Olympic gold medal in Beijing and helped Spain reach the Davis Cup final, where the European team beat Argentina despite the absence of the injured Nadal.
"It has been a wonderful year for Spanish sport, and I am happy to have been part of it,'' Nadal said.
The only other Spaniard to have won the ITF award is Arantxa Sanchez-Vicario. She claimed the women's honor in 1994.
Jankovic finished the year as the top-ranked women's player despite failing to win a Grand Slam title in 2008 - she lost in the U.S. Open final to Serena Williams. But the 23-year-old Serb still managed to win more matches than any other player on tour, and claimed WTA titles in Rome, Beijing, Stuttgart and Moscow.
"I worked really hard this year and to finish No. 1 is where I want to be and where I want to stay,'' Jankovic said.
The ITF selects its world champions based on a system that includes results from the ATP and WTA tours, along with Grand Slams, Olympics and the Davis Cup and Fed Cup.
"Both (Nadal and Jankovic) are excellent ambassadors for their countries, for whom they have achieved considerable success both as individuals and in the ITF team competitions,'' ITF president Francesco Ricci Bitti said.
In men's doubles, Daniel Nestor of Canada and Nenad Zimonjic of Serbia were named ITF World Champions. Cara Black of Zimbabwe and Liezel Huber of the United States took the award for women's doubles.
The ITF Junior World Champion honors went to French Open boys champion Yang Tsung-hua of Taiwan and Wimbledon girls finalist Noppawan Lertcheewakarn of Thailand. Shingo Kunieda of Japan and Esther Vergeer of the Netherlands took the ITF Wheelchair World Champion honors.
The players will receive their awards at the ITF World Champions Dinner on June 2 in Paris.
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