It is Ahmadinejad's first visit to Dushanbe since taking office in August and is part of his two-country Central Asian tour that began in Turkmenistan.
Two major projects to bolster Tajikistan's infrastructure highlight Iranian-Tajik cooperation. President Ahmadinejad is expected to attend a ceremony to open a 5-kilometer tunnel through the Anzob Pass, connecting the capital with the northern Tajik city of Khojand, that was constructed with Iranian assistance.
Tehran has also pledged to invest up to $180 million in a Tajik hydroelectric power plant. Construction of the Sangtuda-2 facility was officially launched in February, although Iran is reportedly seeking additional financial guarantees from the Tajik side.
In Dushanbe, political analyst Ismoil Rahmatov told RFE/RL's Tajik Service that cooperation between the two countries is considerable -- and it is gaining momentum.
"Cooperation between Tajikistan and the Islamic Republic of Iran has been increasing lately. [Iran] has built the Anzob tunnel. The Iranian side has also committed itself to building a number of roads [and] the Sangtuda hydroelectric power plant, and to set up a number of small companies that will put out locally produced [Tajik] goods. At the present stage, Iran is playing a greater [economic] role than any other country in the region."
History Of Engagement
Tehran has shown an interest in Tajikistan since that post-Soviet republic's first days of independence. When many other countries scaled back embassy staffing as Tajikistan's civil war broke out in the early 1990s, Iran increased its diplomatic presence.
The Iranian government provided financial aid to the Tajik government during some of its darkest days of civil war (1992-97).But it also provided safe haven to some of the Islamic leaders from the United Tajik Opposition that was battling Tajik government forces.
Iran -- along with Russia -- played a key role in facilitating a Tajik peace deal in June 1997.
Iran's ambassador to Tajikistan, Nasser Sarmadi Parsa, outlined Ahmadinejad's agenda for his Tajik visit at a July 20 news briefing.
"Private and official discussions with the president of Tajikistan will take place. Also, there will be a meeting between economic delegations of both countries on bilateral issues. There will naturally be some agreements signed, at least five documents," he said. "In the trilateral section [of the visit], it has been decided that President Karzai will attend discussions and there will be at least five [other] documents signed in the three-way discussions."
Three's No Crowd
The Afghan president is due to join his Tajik and Iranian counterparts on July 26 for a trilateral meeting that has already been postponed twice.
Afghan woes have spilled over onto Tajik and Iranian territory on many occasions in the region's recent history. By most accounts, recent fighting in Afghanistan has affected Tajikistan and Iran less than fighting during Afghanistan's civil war in the 1970s, the Soviet invasion of the 1980s, or Taliban forces' drives west and north in the 1990s. But the Afghan drug trade continues to pose a major problem for both Dushanbe and Tehran.
Karzai is likely to seek additional help from neighboring Tajikistan and Iran to improve transportation routes into his country. And he will probably explore the potential for energy supplies to fuel Afghanistan's recovery.
Dushanbe provides a modest amount of electricity to its Afghan neighbor, and the three leaders are expected to discuss ways to extend Tajikistan's power supply routes into Iran.
Regional security issues are also expected to top the agenda of the tripartite meeting.
The Avesta news agency quoted a Tajik Foreign Ministry statement as saying the three presidents will discuss ways to cope with what it described as today's global "challenges and threats."
(RFE/RL correspondent Rahmadkarimi Davlat contributed to this report from Dushanbe.)