The United Nations High Commissioner for Refugees in Athens on Monday received an envelope containing suspicious powder, the fire department said, after dozens of mystery deliveries to Greek universities and institutions.
A police source told AFP the envelope for the high commissioner was similar to nearly 40 letters sent from India to Greece over recent weeks.
All contained an industrial irritant that provoked some allergic reactions but was not dangerous to health, the authorities said.
According to media reports, some of the letters contained "Islamist content" written in English, but they gave no further detail.
Greece's counter-terrorism unit last week launched an inquiry to try to locate the sender.
On Friday another five suspicious envelopes were intercepted at several post offices and the Thessaly Technological Educational Institute in Larisa.
Contacted by AFP, the UNHCR would not comment on Monday's delivery.