Salah was sentenced to nine months' imprisonment in January. However, the court reduced the sentence to five months on Tuesday. The sentence was set to begin on July 25.
Palestinian sources said the court rejected a petition by Salah's lawyers to delay the implementation of the verdict until the end of the holy month of Ramadan.
Judge Yitzhak Shimoni said, "This is a significant sentence in order to clarify to the defendant the severity of the offense he committed."
The court also ordered Salah to pay 7,500 shekels ($2,000) to the officer he allegedly spit on in February 2007.
The assault, which Salah has always denied, took place during demonstrations that erupted in and around Jerusalem's Old City in February 2007 when Israel embarked on excavations near Al-Aqsa Mosque.
Jamal Zahalkah, an Arab member of Israel's Knesset and Chairman of Balad party, said the sentence against Salah "was politically motivated."
Zahalkah said in a press statement that the ruling "aims at silencing the Arab leaders inside Israel."
Salah has been detained on several occasions, most recently after taking part in a Gaza-bound aid flotilla seized on May 31 in a raid in which Israeli naval commandos killed nine Turkish activists in international waters.
In 2003, Salah was arrested and indicted on charges of contacting a foreign agent, for which he received a two-and-a-half year prison sentence.
Israel's Arab community numbers 1.3 million, about 20 percent of the population. It includes the descendants of the 160,000 Palestinians who remained in Israel after the 1948 war and establishment of the Jewish state.
