261 = 32 x 29.
261 has a representation as a sum of two squares: 261 = 62 + 152.
261 is the only three-digit number n for which 2n - n is a prime number.
261 is a nonagonal number.
Inserting the digit 1 between adjacent digits of 261 gives a prime: 21611.
In the novel Contact by Carl Sagan, Dr. Arroway asks Mr. Kitz, "You wanted to keep the first 261 prime numbers secret?"
Source: Prime Curios!