Here are the top 10 developments in this big story:
The UN court said "a continued stay of execution constitutes an indispensable condition for the effective review and reconsideration of the conviction and sentence of Jadhav." The President of the Court Judge Abdulqawi Ahmed Yusuf read out the verdict in The Hague.
The world court said Pakistan had not informed Jadhav about his rights. "Pakistan deprived India of the right to communicate with and have access to Mr Kulbhushan Sudhir Jadhav, to visit him in detention and to arrange for his legal representation," the UN court noted. There was a three-week delay in informing India about Jadhav's arrest, said the judge.
The verdict was 15 to one in favour of India; the lone dissenter was the judge from Pakistan. Even the judge from China supported India, which can be a significant diplomatic pressure point for Islamabad.
PM Modi, welcoming the verdict, tweeted: "Truth and justice have prevailed. Congratulations to the ICJ for a verdict based on extensive study of facts. I am sure Kulbhushan Jadhav will get justice. Our Government will always work for the safety and welfare of every Indian."
"We note that the Court has directed that Pakistan is under an obligation to inform Jadhav without further delay of his rights and to provide Indian consular officers access to him...We expect Pakistan to implement the directive immediately," said foreign ministry spokesperson Raveesh Kumar.
Harish Salve, who was India's advocate at the ICJ, said if the trial by Pakistan isn't fair, India can always go back to the world court. "If the ICJ verdict is disobeyed, we can take measures including by ways including sanctions. I'm sure Pakistan won't go that way. At least I hope so... If the trial by Pakistan isn't fair, we can go back to ICJ."
The court refused to annul Jadhav's death sentence or order his release and safe passage to India.
49-year-old Jadhav was arrested by Pakistan in March 2016 and accused of espionage, a charge India has rubbished. A year later, he was sentenced to death by a Pakistani military court. Denying India's request for access to Jadhav, Pakistan repeatedly said New Delhi only wanted the information gathered by "its spy". A month after the former officer's death sentence, India took Pakistan to the world court, and the execution was stayed in May 2017.
Pakistan claims its security forces had arrested Mr Jadhav from Balochistan province on March 3, 2016, after he entered the country from Iran. India asserted that he was kidnapped from Iran, where he had business interests after retiring from the Navy.
The global court has no means to enforce its rulings, which are final and without appeal.