Scary moment for Barcelona's Ronald Koeman ... the star soccer coach suffered a profuse nosebleed during a news conference -- and had to cut the talk with reporters short over it.

The bizarre scene all went down Friday when 57-year-old Koeman began bleeding from his nostril while he was trying to help media members preview Barca's Saturday game against Sevilla.

You can see in the clip ... as journalists asked Koeman questions, he began to wipe at his nose.

Suddenly, a streak of blood poured down to his lips ... and Koeman struggled to get it to stop, despite using multiple tissues and paper towels.