The discoverer of radioactivity phenomenon is

The phenomenon of radioactivity was discovered by Henri Becquerel.

Benchmarks: Henri Becquerel discovered radioactivity on February 26, 1896. Physicist Wilhelm Röntgen had recently discovered X-rays; Becquerel thought the two phenomena might be connected, and had designed an experiment of his own.

Marie Curie became the first woman to be awarded the Nobel prize and the first person to obtain two Nobel prizes when she won the prize for the discovery of Polonium and Radium in 1911. Though it was Henri Becquerel that discovered radioactivity, it was Marie Curie who coined the term.