Khatami, who is subject to a domestic media ban because of his support for defeated reformist leaders in a disputed 2009 presidential election, took to YouTube late Sunday to send a message to voters.
"After the successful first step in 2013, this coalition should take the second step for the Majlis (parliament)," he said in a four-minute video, dubbing a joint ticket of reformists and moderates as "a list of hope".
He was referring to incumbent Hassan Rouhani's 2013 presidential election victory which led to a nuclear deal that ended a 13-year standoff with world powers.
In this week's election, Rouhani is looking to overturn the majority in parliament of conservatives who resisted the nuclear deal and have also opposed his broader outreach to the West.
Despite the ban on use of his image or words in Iran's print and broadcast media, Khatami, who served as president from 1997 to 2005, remains an important figure in the pro-Rouhani coalition, the Alliance of Reformists and Government Supporters.
"I suggest that all blocs agree to present the list as the list of hope to the people," he said.
The message was simultaneously distributed on the smart phone messaging app, Telegram.
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Khatami said voters should also support lists headed by Rouhani and former president Ayatollah Akbar Hashemi Rafsanjani in the second election taking place Friday, that of the country's Assembly of Experts.
A powerful committee of 88 clerics, the assembly oversees the work of supreme leader Ayatollah Ali Khamenei and is tasked with electing his successor, should he die during its eight-year term.
"Those who are concerned about the country's interests, progress and removal of threats and limitations should vote for all individuals on both lists," Khatami said.
"I repeat, for all individuals on both lists... and take a step to secure the country's dignity, strengthen its security and stability and improve the reform of its affairs."
In a barely veiled dig at hardliners, Khatami's predecessor, Rafsanjani, said the elections were "a valuable opportunity to prevent institutionalisation of political radicalism and religious extremism in society."
Voters' support for moderates would "prove to the world" that rather than the "extremism of a limited group" the true Islam "avoids war and bloodshed in favour of kindness, peace and brotherhood," said Rafsanjani, who was president from 1989 to 1997.
Rouhani's victory in 2013 was made possible after he won backing from Khatami and Rafsanjani and the three are increasingly seen as allies.
Iran's conservatives have had a low-key campaign so far but they will hold a major rally on Tuesday.
Conservative media outlets have accused Britain of "interfering in Iran's internal affairs", charging that the BBC's Persian service has attempted to sway voters in favour of the reformists.