The use of shrinkage-reducing admixtures (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. Various forms of SRA are commercially available and they may act through different mechanisms. Some SRA mainly acts on drying and weight loss leading to shrinkage. In this paper, the influence of a liquid SRA on plastic shrinkage, long-term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-to-cement ratios (w/c) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1% to the total mass of binder, while keeping the other parameters constant. At early age, the results indicate that the SRA lead to the same plastic shrinkage for w/c = 0.65 while it reduced the plastic shrinkage by 25% for w/c = 0.43. Drying shrinkage was assessed from 1 day on hardened concrete. The SRA reduced the 7 day drying shrinkage for w/c = 0.65 and w/c = 0.43 concrete mixtures by up to 56%–31%, respectively, and the 70 day drying shrinkage by up to 33%–25% when the specimens were cured for 24 h then stored at relative humidity of 50%. At equal water-to-cement ratios, the SRA is seen to be more efficient in reducing drying shrinkage at early ages. These findings suggest that the SRA is most effective when internal relative humidity is relatively high or when higher porosity exists in the material. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 300 to 1000 nm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels where capillary stresses are the main cause of shrinkage. Thus, this phenomenon may contribute in the reduction of drying shrinkage that occurs when this liquid SRA is used in concrete.