Do you need a master's degree to work for the UN?

At a Professional (P-1to5 or D-1-2) level, yes. In some areas, a Ph.D is even a better idea.

However, the U.N is a huge organization. You might want to decide in which area and/or department you want to work before deciding on studies.

Also which kind of job you want. Very-short term missions, 2-3 years contracts, etc... Most jobs at a Professional level are 2-year renewable contracts.

Don't forget that most positions asks for a very high mobility, and that a second or third language is preferred (among the official languages of the U.N)

"The Organization uses six official languages in its intergovernmental meetings and documents, Arabic, Chinese, English, French, Russian and Spanish; the Secretariat uses two working languages, English and French."

For most positions, you will be even required to know or learn English AND French.

Take a look here : http://icsc.un.org/joblinks.asp

You can still look for local positions ( G and L ) which sometimes do not require a Master's degree.

Volunteers are always needed !