Prospex Marinemaster 300

Name: Seiko Prospex Marinemaster 300
Brand: Seiko
SKU: SBDX017

SBDX017

The Marinemaster 300 at its most purposeful — a monobloc diver built to last.

Estimated market value

$2,300

Confidence: High

An estimate from current market data — not an offer. Your actual value depends on condition, completeness, and authentication.

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Overview

The Seiko SBDX017 is a professional-grade diver within the long-running Prospex Marinemaster 300 line, pairing a 44.3 mm stainless steel monobloc case with the in-house Cal. 8L35 automatic movement. Its black dial, dressed with bold luminous markers, prioritises legibility under demanding conditions. Produced between 2015 and 2018, the reference occupies a respected position in Seiko's hierarchy of serious tool watches.

History

Seiko's Marinemaster lineage traces back to the brand's decades-long commitment to professional diving instruments, a tradition that predates the Prospex name itself. The SBDX017 arrived as part of a generation of Marinemaster 300 references that refined the formula established by earlier models, retaining the distinctive monobloc case architecture that defines the series. Its production run of roughly three years places it among the mid-2010s Prospex references that helped consolidate the line's reputation among serious collectors. The reference was succeeded by subsequent Marinemaster iterations that continued to evolve the design language.

Notable points

The monobloc case construction — in which the case and caseback are formed from a single piece of steel — is a hallmark of the Marinemaster 300 series and contributes significantly to its water-resistance integrity.
Cal. 8L35 is an in-house Seiko automatic movement, reflecting the brand's longstanding practice of vertical integration in both movement design and manufacture.
At 44.3 mm, the case diameter sits at a commanding size well suited to dive-watch proportions, while remaining consistent with the Marinemaster 300 family's established dimensions.
The black dial with luminous markers follows a strictly functional brief, prioritising underwater legibility over decorative considerations — a philosophy central to the Prospex line.
Produced for approximately three years, the SBDX017 has become a sought-after reference among Prospex collectors who value the combination of the monobloc case and the 8L35 calibre.