Tsvangirai delayed his return to Zimbabwe on Saturday after his party said it received warnings about the alleged plot.
Biti said Tsvangirai will return "very soon".
The MDC's leader had left the county over security concerns following Zimbabwe's disputed elections on March 29.
The party won a majority in the parliamentary elections and Tsvangirai also claimed victory in the presidential election.
But the country's election commission said he had not won enough votes for an outright victory, prompting a run-off vote for June 27 against Robert Mugabe, Zimbabwe's president.
Mugabe's government said it was unaware of any plot and the allegation was simply a "media stunt".
Bright Matonga, Zimbabwe's deputy information minister said: "Like I've said before, we know he did not leave the country on security grounds.
"We are not aware of any plot against anyone, but we're sure he's playing to the international media gallery."
Zimbabwe's election process has seen repeated delays, violence and allegations of electoral fraud.
Tsvangirai accuses Robert Mugabe, Zimbabwe's president, of delaying the run-off to mount a campaign of intimidation against his supporters.