A bullet and sympathy card have been sent to an SDLP Assembly member, the party has said.
The apparent death threat was received by South Belfast representative Conall McDevitt on Wednesday morning, said an SDLP spokeswoman.
The MLA has been subject to a series of threats during the ongoing Union flag furore.
Other SDLP members have been targeted amid the controversy surrounding Belfast City Council's decision to limit the number of days the flag flies over City Hall.
Members of other political parties have also received death threats since the council's vote in December.
Mr McDevitt said he would not be deterred by the development.
"These threats only strengthen my determination that we must work towards a better future for everyone in this region, and will not prevent me from doing the job I have been democratically elected to do," he said.
"The foundations for a new society laid by the Good Friday Agreement will never be undermined by those who seek to drag us back to violence.
"These are the acts of fascists and they will be defeated democratically."