Aleatico is an Italian red winegrape variety traditionally used partially dehydrated to produce Passito wines. The aim of this research is to assess the effect of two different off-vine withering techniques (natural sun-exposed: SUN, and controlled temperature and humidity conditions: CTR) on grapes physicochemical modifications, focusing on mechanical properties, phenolic profile, and cell-wall polysaccharide structures. The composition and extractability of polyphenols were studied before and after the two withering processes, differentiating the tissues (skin, pulp, seeds). A stronger effect on cell-wall polysaccharides composition pectin was highlighted by chemical and immunochemical approaches in SUN skins at the level of pectin chains and de-esterification, affecting grapes mechanical properties, especially for skin rigidity (<E_sk). Sun-dehydrated grapes showed the highest pH (about 0.3 units more than FRESH and CTR), and the lowest acidity (−0.71 g/L vs FRESH) and anthocyanin content in wine-like solution (−41%), whereas these parameters were better preserved in controlled conditions. Instead, seeds tannins were higher in SUN than other samples, with a lower oligomers/polymers ratio. Indeed, after withering the phenolic extractability decreased in skins (total phenolics: from 60% for FRESH to 40% for SUN; anthocyanins: from 72% for FRESH to 46% for SUN) and increased in seeds (total phenolics: from 30% for FRESH to 41% for SUN) compared with fresh samples, in both cases more markedly for SUN than CTR. Skin phenolic composition after 10 days of simulated-maceration was affected by the combination of the loss/concentration balance, together with their modified release in the wine-like solution. As regards seeds, instead, the differences highlighted by the extractable profiles were due to the modified extractability as consequence of the different withering conditions and not to chemical quantitative/qualitative modifications of phenolic compounds caused by dehydration.