The bridge, opened by Tajik President Imomali Rakhmonov and Prince Aga Khan IV, leader of Shi'ite Ismaili Muslims, is the first vehicle bridge spanning the Pyandzh river between the countries.
Vehicles can now be driven from the Afghan capital Kabul through north-eastern Afghanistan to Khorog in Tajikistan, then on to Kyrgyzstan and Russia.
"The opening of this new bridge is a significant step forward," the prince, who represents more than 20 million Ismailis around the world, told Reuters in Dushanbe on Saturday, before leaving for the bridge.
Goods can now be driven "through Kyrgyzstan and Khorog south to Afghanistan," he said. "That is, you will have a direct route from north to south which previously did not exist."
The bridge has been largely funded by the philanthropic prince, the 49th direct descendant of the Prophet Mohammed, who has given more than $30 million in aid to various projects in Tajikistan since 1994.
No cost estimate for the new bridge was available from the Aga Khan Foundation.
A bridge already crosses the Pyandzh river at Termez in Uzbekistan, which gives access to the Afghan city of Mazar-i-Sharif. The Soviet army fled Afghanistan via that route after its 10-year occupation in 1989.
Much of the traffic between Afghanistan and the outside world had up to now moved across the Khyber Pass to Pakistan.
Tajikistan, the poorest of the former Soviet republics, was plunged into civil war from 1992 to 1997, shortly after the Soviet Union collapsed. Its average wages are still about $9 per month.