In a major boost to India, which has moved the UN's highest judicial body against the death sentence, the ICJ instructed Pakistan to take all "necessary measures at its disposal" to ensure that Jadhav was not executed pending a final decision by it.
The decision of the 11-judge bench was unanimous, ICJ President Ronny Abraham said while reading out the verdict.
The ICJ said that India should have been granted consular access to its national as per the Vienna Convention to which both countries have been signatories since 1977.
The ICJ asserted its jurisdiction over the case of the 46-year-old former Indian Navy officer while noting that the circumstances of his arrest remain disputed.
"The 150-day period for clemency given by Pakistan, which ends in August suggests that the execution can happen immediately thereafter," observed the international court in The Hague.
The ruling came three days after India and Pakistan gave their submissions in the case.
Pakistan claims its security forces arrested Jadhav from its restive Balochistan province on March 3 last year after he reportedly entered from Iran. However, India maintains that he was kidnapped from Iran where he had business interests after retiring from the Indian Navy.
Jadhav's case is the latest flashpoint in the tensions between Pakistan and India.
On May 8, India moved the ICJ against the death penalty, alleging violation of the Vienna Convention on Consular Relations. On May 9, the global court stayed the death sentence as a provisional measure.