Siewers, who joined the business in May, will now be responsible for the all brand's activities in the UK and EMEA markets.
The UK-based leadership team will now report directly to Siewers, as the brand looks to build on its strategic growth in these regions.
The international market will continue to be managed and supported by Stuart Pearce, UK and Ireland sales director; Lee Surman, director of partnerships and trading; and Natalie Holder, international marketing manager, to ensure "continuity and momentum across the cruise line’s operations".
Azamara chief executive Dondra Ritzenthaler said: “We are truly grateful for David Duff’s commitment to our business during the 17 years he has been with the brand. We extend our heartfelt gratitude to him for his significant contributions and wish him every success in his future endeavours."
Duff added: “I am proud of what we have achieved together at Azamara Cruises, from growing our presence across the UK and EMEA, to cementing our reputation as the leader in Destination Immersion cruising.
"It has been a privilege to work with such talented colleagues and dedicated partners, and I am confident that the business will continue to thrive.”
Siewers continued: “I am truly grateful for all that David Duff has established. With a talented local sales team and UK-based call centre, UK and EMEA remains a priority focus for Azamara Cruises as we look ahead to exciting new initiatives.”