Chlorophyll determination with a portable chlorophyll meter can indicate the period of highest N demand of plants and whether sidedressing is required or not. Real time sensing of plant nitrogen status can be a useful tool in managing nitrogen inputs. The purpose of this study was to evaluate the efficiency of nitrogen sufficiency index (NSI) calculated based on the relative chlorophyll index (RCI) in the third fully expanded trifoliolate leaf from the top, measured with a portable chlorophyll meter, as an indicator of time of N sidedressing fertilization and which NSI value, 90%, 93% and 95%, is the most appropriate to indicate the moment of N fertilization on common bean cultivar IPR Eldorado. The experiment was carried out in the dry growing season of the agricultural year 2012 on a dystroferric Red Nitosol, in Botucatu, São Paulo State, Brazil.  The experiment was arranged in a randomized complete block design with six treatments, consisting of N managements (M1: 200 kg ha^-1 N (40 kg at sowing + 80 kg at 10 days after emergence (DAE) + 80 kg 20 DAE ); M2: 100 kg ha^-1 N (20 kg at sowing + 40 kg at 10 DAE + 40 kg at 20 DAE); M3: 20 kg ha^-1 N at sowing + 30 kg ha^-1 when chlorophyll meter readings indicated NSI < 96%; M4: 20 kg ha^-1 N at sowing + 30 kg ha^-1 N when chlorophyll meter readings indicated NSI < 93 %; M5: 20 kg ha^-1 N at sowing + 30 kg ha^-1 when chlorophyll meter readings indicated NSI < 90% and M6: control (without N application)) and four replications. The variables RCI, aboveground dry matter, total leaf N concentration, production components, grain yield, relative yield, and N use efficiency were evaluated. By monitoring the RCI with the chlorophyll meter, the period of N sidedressing of common bean could be defined, improving N use efficiency and avoiding unnecessary N supply to common bean. The NSI<90% of the reference area was more efficient to define the moment of N sidedressing of common bean, to increase N use efficiency.