The Rota do Guadiana follows the Guadiana River through one of the most sparsely populated corners of southern Portugal, and the silence out here is the first thing that gets you.
Mértola itself is a proper starting point — a whitewashed medieval town draped over a hill above the river, with a castle that doubles as a Moorish mosque and a small museum worth an hour of your time before the wheels start turning. From here the route traces roughly 130 kilometres southward toward Pomarão, though most riders reverse that and pedal north to finish dramatically at Mértola's ramparts.
The terrain is genuinely easy by cycling standards, with rolling Alentejo scrubland keeping things interesting without punishing you. Expect some unpaved stretches of compacted dirt and gravel — a hybrid or gravel tyre handles it far better than a road bike.
The river appears and vanishes as the route dips between cork oak and eucalyptus, and you will almost certainly see black storks, purple herons, and white-headed ducks if you stop and look. Traffic is minimal to non-existent on most sections, which is part of the appeal and part of the challenge — services are genuinely scarce, so carry more water and food than you think you need.
Two comfortable days work well, with an overnight in the hamlet of Alcoutim or a rural quinta somewhere mid-route. Bike hire is available in Mértola through local operators, though supply is limited, so book ahead. Accommodation outside Mértola is sparse; wild camping is technically restricted, but registered rural guesthouses exist if you research beforehand.
Go between October and April — summer heat in the Alentejo regularly tops 40°C, which turns a gentle ride into something genuinely dangerous.