For annual and perennial crops, mathematical models have been developed to describe tissue nitrogen (N) dilution during crop growth and to estimate the plant N status applying the N nutrition index (NNI), the ratio between the actual tissue N concentration ([N]) and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (critical tissue N concentration, [N]c). The relationship between shoot [N]c and shoot dry matter (DM, t ha−1) can be described by an allometric power equation: [N]c = a DM−b, where a and b are crop-specific parameters. Critical N dilution curves (CNDC) have been determined for several C3 crops but not specifically for sunflower (Helianthus annuus L.). The objectives of this work were to (i) determine and validate the N dilution curves for critical, minimum, and maximum [N] for sunflower from the juvenile stages to the end of flowering, (ii) compare the critical curve with published CNDCs for other C3 crops, and (iii) estimate the range of variation of NNI for different levels of N fertilization and irrigation. A wide range of field experiments from Argentina, Australia, France, Italy, and Spain was used to establish the dilution curve for sunflower and to independently validate it. The fitted CNDC [N]c = 4.53 DM−0.42 yielded lower values for [N]c than references used until now for diagnosis and decision making in sunflower. The value of parameter a was generally similar to that of other C3 species, but the value for parameter b differed. This was possibly associated with species differences in dry mass partitioning, and justified the development of a sunflower-specific CNDC. A preliminary reference curve for maximum [N] suggested an evolution from the juvenile stages to the end of flowering similar to that of [N]c. Minimum [N], in contrast, appeared to be more constant over time. Relationships between relative grain yield and NNI across a range of locations indicated that in general, maximum grain yield was reached around NNI = 0.8, although at one location this was around NNI = 1.0. The CNDC can provide useful applications for crop modeling, N status diagnosis, and N fertilization decision.