Roberto Vila, 34, was snatched by four armed men from a car near the town of Deir Al-Balah in central Gaza and bundled into a yellow Skoda, witnesses and security officials said.
The gunmen released Vila's French female colleague and two Palestinian assistants who were also in the car, they said. Police said they questioned the women and launched a search for the perpetrators.
Vila worked for the media department of the Spanish aid group Co-operation Assembly for Peace, which said in a statement that it was "confident" that he would be released soon.
There was no immediate claim of responsibility for the abduction.
Palestinian prime minister Ismail Haniya, slammed the abduction and demanded that the aid worker be released.
"We reject this kidnapping. It is immoral," he told reporters in Gaza City. "I ask the people who kidnapped him to release him immediately. This is bad for Palestinian people."
Haniya said that he had ordered the interior ministry to work toward the release of the Spaniard.
The Al-Aqsa Brigades, a militant group close to president Mahmud Abbas's Fatah party, also condemned the abduction.
"We strongly condemn the kidnapping and we're going to make all efforts to release him," a spokesman told AFP.
Monday's abduction came a week after another Spaniard, a photographer for the US-based Associated Press news agency, was held by Gaza gunmen for around 15 hours before being released unharmed.
Emilio Morenatti, 37, was seized by four masked and armed men as he left his residence in the center of Gaza City early on October 24. He was released late Tuesday, with no one claiming responsibility for the abduction.
Kidnappings of foreigners have increased in the impoverished Palestinian territory, which has been seized by an unprecedented financial and political crisis for months.
Most of the foreigners have been released unharmed within days.
In August, two journalists for the US Fox News television network spent 13 days in captivity of a previously unknown militant group, Holy Jihad Brigades, who demanded the release of all Muslim prisoners held in US jails.
It marked the first time that kidnappers had made a demand on a foreign government, as usually Palestinian abductors seize foreigners to obtain concessions from the Palestinian Authority.
The two Fox reporters, US reporter Steve Centanni and New Zealand freelance cameraman Olaf Wiig, were released after being forced to convert to Islam at gunpoint, with a videotape of their conversion issued just hours before they were freed.
The territories have been gripped by an unprecedented political and financial crisis that followed the rise to power of the Hamas movement in January's election, the subsequent Western aid cut-off and internecine Palestinian violence.
Kidnappings have been particularly prevalent in the Gaza Strip, where armed groups and clans have proliferated following Israel's withdrawal from the territory last year after a 38-year occupation.
The chaos has been fed by a massive presence of arms in Gaza, inter-factional fighting between Hamas and Fatah and a four-month Israeli offensive.

Copyright AFP (Agence France-Presse), 2006