"President Assad is part of the solution," de Mistura told a joint press conference with Austrian Foreign Minister Sebastian Kurz.
"I will continue to have very important discussions with him," he added, noting that "the only solution is a political solution."
De Mistura, who was in Damascus this week where he met with Assad, is due to deliver a report on his mission to the UN Security Council on February 17.
If no solution to the conflict is found, "the only one who takes advantage of it is (the Islamic State group) Isis Daesh," de Mistura said, referring to the jihadists who have taken over parts of Syria and Iraq.
The group is a "monster waiting for this conflict to take place in order to be able to take advantage," he said.
Kurz meanwhile agreed that "in the fight against Isis it can be necessary to fight on the same side" but insisted that "Assad will never be a friend or even a partner."
Human rights groups have accused Syria's government of indiscriminate bombardment of civilians in rebel-held areas, including with crude "barrel bombs" — allegations Assad denied in a BBC interview this week.
In the interview, he also complained that in the fight against IS, "there is no dialogue" with the US-led coalition, which began airstrikes in September.
"There's, let's say, information, but not dialogue," the embattled leader said.
In a poll on Thursday, 53 percent of residents in opposition-held areas of Syria's second city of Aleppo — which has seen some of the country's worst violence since July 2012 — said they favoured de Mistura's October proposal of a "freeze" in fighting.
But a great majority also said they were sceptical that a truce would hold.
Syria's war, which began as peaceful protests in March 2011, has since killed more than 210,000 people, with regime troops shelling rebel-held areas almost daily. Several rounds of negotiations have ended in failure.